/* @internal */ // Don't expose that we use this
// Based on lib.es6.d.ts
interface PromiseConstructor {
    new <T>(executor: (resolve: (value?: T | PromiseLike<T>) => void, reject: (reason?: any) => void) => void): Promise<T>;
    reject(reason: any): Promise<never>;
    all<T>(values: (T | PromiseLike<T>)[]): Promise<T[]>;
}
/* @internal */
declare var Promise: PromiseConstructor; // eslint-disable-line no-var

/* @internal */
namespace ts {
    // These utilities are common to multiple language service features.
    //#region
    export const scanner: Scanner = createScanner(ScriptTarget.Latest, /*skipTrivia*/ true);

    export const enum SemanticMeaning {
        None = 0x0,
        Value = 0x1,
        Type = 0x2,
        Namespace = 0x4,
        All = Value | Type | Namespace
    }

    export function getMeaningFromDeclaration(node: Node): SemanticMeaning {
        switch (node.kind) {
            case SyntaxKind.VariableDeclaration:
                return isInJSFile(node) && getJSDocEnumTag(node) ? SemanticMeaning.All : SemanticMeaning.Value;

            case SyntaxKind.Parameter:
            case SyntaxKind.BindingElement:
            case SyntaxKind.PropertyDeclaration:
            case SyntaxKind.PropertySignature:
            case SyntaxKind.PropertyAssignment:
            case SyntaxKind.ShorthandPropertyAssignment:
            case SyntaxKind.MethodDeclaration:
            case SyntaxKind.MethodSignature:
            case SyntaxKind.Constructor:
            case SyntaxKind.GetAccessor:
            case SyntaxKind.SetAccessor:
            case SyntaxKind.FunctionDeclaration:
            case SyntaxKind.FunctionExpression:
            case SyntaxKind.ArrowFunction:
            case SyntaxKind.CatchClause:
            case SyntaxKind.JsxAttribute:
                return SemanticMeaning.Value;

            case SyntaxKind.TypeParameter:
            case SyntaxKind.InterfaceDeclaration:
            case SyntaxKind.TypeAliasDeclaration:
            case SyntaxKind.TypeLiteral:
                return SemanticMeaning.Type;

            case SyntaxKind.JSDocTypedefTag:
                // If it has no name node, it shares the name with the value declaration below it.
                return (node as JSDocTypedefTag).name === undefined ? SemanticMeaning.Value | SemanticMeaning.Type : SemanticMeaning.Type;

            case SyntaxKind.EnumMember:
            case SyntaxKind.ClassDeclaration:
                return SemanticMeaning.Value | SemanticMeaning.Type;

            case SyntaxKind.ModuleDeclaration:
                if (isAmbientModule(<ModuleDeclaration>node)) {
                    return SemanticMeaning.Namespace | SemanticMeaning.Value;
                }
                else if (getModuleInstanceState(node as ModuleDeclaration) === ModuleInstanceState.Instantiated) {
                    return SemanticMeaning.Namespace | SemanticMeaning.Value;
                }
                else {
                    return SemanticMeaning.Namespace;
                }

            case SyntaxKind.EnumDeclaration:
            case SyntaxKind.NamedImports:
            case SyntaxKind.ImportSpecifier:
            case SyntaxKind.ImportEqualsDeclaration:
            case SyntaxKind.ImportDeclaration:
            case SyntaxKind.ExportAssignment:
            case SyntaxKind.ExportDeclaration:
                return SemanticMeaning.All;

            // An external module can be a Value
            case SyntaxKind.SourceFile:
                return SemanticMeaning.Namespace | SemanticMeaning.Value;
        }

        return SemanticMeaning.All;
    }

    export function getMeaningFromLocation(node: Node): SemanticMeaning {
        if (node.kind === SyntaxKind.SourceFile) {
            return SemanticMeaning.Value;
        }
        else if (node.parent.kind === SyntaxKind.ExportAssignment || node.parent.kind === SyntaxKind.ExternalModuleReference) {
            return SemanticMeaning.All;
        }
        else if (isInRightSideOfInternalImportEqualsDeclaration(node)) {
            return getMeaningFromRightHandSideOfImportEquals(node as Identifier);
        }
        else if (isDeclarationName(node)) {
            return getMeaningFromDeclaration(node.parent);
        }
        else if (isTypeReference(node)) {
            return SemanticMeaning.Type;
        }
        else if (isNamespaceReference(node)) {
            return SemanticMeaning.Namespace;
        }
        else if (isTypeParameterDeclaration(node.parent)) {
            Debug.assert(isJSDocTemplateTag(node.parent.parent)); // Else would be handled by isDeclarationName
            return SemanticMeaning.Type;
        }
        else if (isLiteralTypeNode(node.parent)) {
            // This might be T["name"], which is actually referencing a property and not a type. So allow both meanings.
            return SemanticMeaning.Type | SemanticMeaning.Value;
        }
        else {
            return SemanticMeaning.Value;
        }
    }

    function getMeaningFromRightHandSideOfImportEquals(node: Node): SemanticMeaning {
        //     import a = |b|; // Namespace
        //     import a = |b.c|; // Value, type, namespace
        //     import a = |b.c|.d; // Namespace
        const name = node.kind === SyntaxKind.QualifiedName ? node : isQualifiedName(node.parent) && node.parent.right === node ? node.parent : undefined;
        return name && name.parent.kind === SyntaxKind.ImportEqualsDeclaration ? SemanticMeaning.All : SemanticMeaning.Namespace;
    }

    export function isInRightSideOfInternalImportEqualsDeclaration(node: Node) {
        while (node.parent.kind === SyntaxKind.QualifiedName) {
            node = node.parent;
        }
        return isInternalModuleImportEqualsDeclaration(node.parent) && node.parent.moduleReference === node;
    }

    function isNamespaceReference(node: Node): boolean {
        return isQualifiedNameNamespaceReference(node) || isPropertyAccessNamespaceReference(node);
    }

    function isQualifiedNameNamespaceReference(node: Node): boolean {
        let root = node;
        let isLastClause = true;
        if (root.parent.kind === SyntaxKind.QualifiedName) {
            while (root.parent && root.parent.kind === SyntaxKind.QualifiedName) {
                root = root.parent;
            }

            isLastClause = (<QualifiedName>root).right === node;
        }

        return root.parent.kind === SyntaxKind.TypeReference && !isLastClause;
    }

    function isPropertyAccessNamespaceReference(node: Node): boolean {
        let root = node;
        let isLastClause = true;
        if (root.parent.kind === SyntaxKind.PropertyAccessExpression) {
            while (root.parent && root.parent.kind === SyntaxKind.PropertyAccessExpression) {
                root = root.parent;
            }

            isLastClause = (<PropertyAccessExpression>root).name === node;
        }

        if (!isLastClause && root.parent.kind === SyntaxKind.ExpressionWithTypeArguments && root.parent.parent.kind === SyntaxKind.HeritageClause) {
            const decl = root.parent.parent.parent;
            return (decl.kind === SyntaxKind.ClassDeclaration && (<HeritageClause>root.parent.parent).token === SyntaxKind.ImplementsKeyword) ||
                (decl.kind === SyntaxKind.InterfaceDeclaration && (<HeritageClause>root.parent.parent).token === SyntaxKind.ExtendsKeyword);
        }

        return false;
    }

    function isTypeReference(node: Node): boolean {
        if (isRightSideOfQualifiedNameOrPropertyAccess(node)) {
            node = node.parent;
        }

        switch (node.kind) {
            case SyntaxKind.ThisKeyword:
                return !isExpressionNode(node);
            case SyntaxKind.ThisType:
                return true;
        }

        switch (node.parent.kind) {
            case SyntaxKind.TypeReference:
                return true;
            case SyntaxKind.ImportType:
                return !(node.parent as ImportTypeNode).isTypeOf;
            case SyntaxKind.ExpressionWithTypeArguments:
                return !isExpressionWithTypeArgumentsInClassExtendsClause(<ExpressionWithTypeArguments>node.parent);
        }

        return false;
    }

    export function isCallExpressionTarget(node: Node, includeElementAccess = false, skipPastOuterExpressions = false): boolean {
        return isCalleeWorker(node, isCallExpression, selectExpressionOfCallOrNewExpressionOrDecorator, includeElementAccess, skipPastOuterExpressions);
    }

    export function isNewExpressionTarget(node: Node, includeElementAccess = false, skipPastOuterExpressions = false): boolean {
        return isCalleeWorker(node, isNewExpression, selectExpressionOfCallOrNewExpressionOrDecorator, includeElementAccess, skipPastOuterExpressions);
    }

    export function isCallOrNewExpressionTarget(node: Node, includeElementAccess = false, skipPastOuterExpressions = false): boolean {
        return isCalleeWorker(node, isCallOrNewExpression, selectExpressionOfCallOrNewExpressionOrDecorator, includeElementAccess, skipPastOuterExpressions);
    }

    export function isTaggedTemplateTag(node: Node, includeElementAccess = false, skipPastOuterExpressions = false): boolean {
        return isCalleeWorker(node, isTaggedTemplateExpression, selectTagOfTaggedTemplateExpression, includeElementAccess, skipPastOuterExpressions);
    }

    export function isDecoratorTarget(node: Node, includeElementAccess = false, skipPastOuterExpressions = false): boolean {
        return isCalleeWorker(node, isDecorator, selectExpressionOfCallOrNewExpressionOrDecorator, includeElementAccess, skipPastOuterExpressions);
    }

    export function isJsxOpeningLikeElementTagName(node: Node, includeElementAccess = false, skipPastOuterExpressions = false): boolean {
        return isCalleeWorker(node, isJsxOpeningLikeElement, selectTagNameOfJsxOpeningLikeElement, includeElementAccess, skipPastOuterExpressions);
    }

    function selectExpressionOfCallOrNewExpressionOrDecorator(node: CallExpression | NewExpression | Decorator) {
        return node.expression;
    }

    function selectTagOfTaggedTemplateExpression(node: TaggedTemplateExpression) {
        return node.tag;
    }

    function selectTagNameOfJsxOpeningLikeElement(node: JsxOpeningLikeElement) {
        return node.tagName;
    }

    function isCalleeWorker<T extends CallExpression | NewExpression | TaggedTemplateExpression | Decorator | JsxOpeningLikeElement>(node: Node, pred: (node: Node) => node is T, calleeSelector: (node: T) => Expression, includeElementAccess: boolean, skipPastOuterExpressions: boolean) {
        let target = includeElementAccess ? climbPastPropertyOrElementAccess(node) : climbPastPropertyAccess(node);
        if (skipPastOuterExpressions) {
            target = skipOuterExpressions(target);
        }
        return !!target && !!target.parent && pred(target.parent) && calleeSelector(target.parent) === target;
    }

    export function climbPastPropertyAccess(node: Node) {
        return isRightSideOfPropertyAccess(node) ? node.parent : node;
    }

    export function climbPastPropertyOrElementAccess(node: Node) {
        return isRightSideOfPropertyAccess(node) || isArgumentExpressionOfElementAccess(node) ? node.parent : node;
    }

    export function getTargetLabel(referenceNode: Node, labelName: string): Identifier | undefined {
        while (referenceNode) {
            if (referenceNode.kind === SyntaxKind.LabeledStatement && (<LabeledStatement>referenceNode).label.escapedText === labelName) {
                return (<LabeledStatement>referenceNode).label;
            }
            referenceNode = referenceNode.parent;
        }
        return undefined;
    }

    export function hasPropertyAccessExpressionWithName(node: CallExpression, funcName: string): boolean {
        if (!isPropertyAccessExpression(node.expression)) {
            return false;
        }

        return node.expression.name.text === funcName;
    }

    export function isJumpStatementTarget(node: Node): node is Identifier & { parent: BreakOrContinueStatement } {
        return isIdentifier(node) && tryCast(node.parent, isBreakOrContinueStatement)?.label === node;
    }

    export function isLabelOfLabeledStatement(node: Node): node is Identifier {
        return isIdentifier(node) && tryCast(node.parent, isLabeledStatement)?.label === node;
    }

    export function isLabelName(node: Node): boolean {
        return isLabelOfLabeledStatement(node) || isJumpStatementTarget(node);
    }

    export function isTagName(node: Node): boolean {
        return tryCast(node.parent, isJSDocTag)?.tagName === node;
    }

    export function isRightSideOfQualifiedName(node: Node) {
        return tryCast(node.parent, isQualifiedName)?.right === node;
    }

    export function isRightSideOfPropertyAccess(node: Node) {
        return tryCast(node.parent, isPropertyAccessExpression)?.name === node;
    }

    export function isArgumentExpressionOfElementAccess(node: Node) {
        return tryCast(node.parent, isElementAccessExpression)?.argumentExpression === node;
    }

    export function isNameOfModuleDeclaration(node: Node) {
        return tryCast(node.parent, isModuleDeclaration)?.name === node;
    }

    export function isNameOfFunctionDeclaration(node: Node): boolean {
        return isIdentifier(node) && tryCast(node.parent, isFunctionLike)?.name === node;
    }

    export function isLiteralNameOfPropertyDeclarationOrIndexAccess(node: StringLiteral | NumericLiteral | NoSubstitutionTemplateLiteral): boolean {
        switch (node.parent.kind) {
            case SyntaxKind.PropertyDeclaration:
            case SyntaxKind.PropertySignature:
            case SyntaxKind.PropertyAssignment:
            case SyntaxKind.EnumMember:
            case SyntaxKind.MethodDeclaration:
            case SyntaxKind.MethodSignature:
            case SyntaxKind.GetAccessor:
            case SyntaxKind.SetAccessor:
            case SyntaxKind.ModuleDeclaration:
                return getNameOfDeclaration(<Declaration>node.parent) === node;
            case SyntaxKind.ElementAccessExpression:
                return (<ElementAccessExpression>node.parent).argumentExpression === node;
            case SyntaxKind.ComputedPropertyName:
                return true;
            case SyntaxKind.LiteralType:
                return node.parent.parent.kind === SyntaxKind.IndexedAccessType;
            default:
                return false;
        }
    }

    export function isExpressionOfExternalModuleImportEqualsDeclaration(node: Node) {
        return isExternalModuleImportEqualsDeclaration(node.parent.parent) &&
            getExternalModuleImportEqualsDeclarationExpression(node.parent.parent) === node;
    }

    export function getContainerNode(node: Node): Declaration | undefined {
        if (isJSDocTypeAlias(node)) {
            // This doesn't just apply to the node immediately under the comment, but to everything in its parent's scope.
            // node.parent = the JSDoc comment, node.parent.parent = the node having the comment.
            // Then we get parent again in the loop.
            node = node.parent.parent;
        }

        while (true) {
            node = node.parent;
            if (!node) {
                return undefined;
            }
            switch (node.kind) {
                case SyntaxKind.SourceFile:
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.MethodSignature:
                case SyntaxKind.FunctionDeclaration:
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                case SyntaxKind.ClassDeclaration:
                case SyntaxKind.InterfaceDeclaration:
                case SyntaxKind.EnumDeclaration:
                case SyntaxKind.ModuleDeclaration:
                    return <Declaration>node;
            }
        }
    }

    export function getNodeKind(node: Node): ScriptElementKind {
        switch (node.kind) {
            case SyntaxKind.SourceFile:
                return isExternalModule(<SourceFile>node) ? ScriptElementKind.moduleElement : ScriptElementKind.scriptElement;
            case SyntaxKind.ModuleDeclaration:
                return ScriptElementKind.moduleElement;
            case SyntaxKind.ClassDeclaration:
            case SyntaxKind.ClassExpression:
                return ScriptElementKind.classElement;
            case SyntaxKind.InterfaceDeclaration: return ScriptElementKind.interfaceElement;
            case SyntaxKind.TypeAliasDeclaration:
            case SyntaxKind.JSDocCallbackTag:
            case SyntaxKind.JSDocTypedefTag:
                return ScriptElementKind.typeElement;
            case SyntaxKind.EnumDeclaration: return ScriptElementKind.enumElement;
            case SyntaxKind.VariableDeclaration:
                return getKindOfVariableDeclaration(<VariableDeclaration>node);
            case SyntaxKind.BindingElement:
                return getKindOfVariableDeclaration(<VariableDeclaration>getRootDeclaration(node));
            case SyntaxKind.ArrowFunction:
            case SyntaxKind.FunctionDeclaration:
            case SyntaxKind.FunctionExpression:
                return ScriptElementKind.functionElement;
            case SyntaxKind.GetAccessor: return ScriptElementKind.memberGetAccessorElement;
            case SyntaxKind.SetAccessor: return ScriptElementKind.memberSetAccessorElement;
            case SyntaxKind.MethodDeclaration:
            case SyntaxKind.MethodSignature:
                return ScriptElementKind.memberFunctionElement;
            case SyntaxKind.PropertyAssignment:
                const {initializer} = node as PropertyAssignment;
                return isFunctionLike(initializer) ? ScriptElementKind.memberFunctionElement : ScriptElementKind.memberVariableElement;
            case SyntaxKind.PropertyDeclaration:
            case SyntaxKind.PropertySignature:
            case SyntaxKind.ShorthandPropertyAssignment:
            case SyntaxKind.SpreadAssignment:
                return ScriptElementKind.memberVariableElement;
            case SyntaxKind.IndexSignature: return ScriptElementKind.indexSignatureElement;
            case SyntaxKind.ConstructSignature: return ScriptElementKind.constructSignatureElement;
            case SyntaxKind.CallSignature: return ScriptElementKind.callSignatureElement;
            case SyntaxKind.Constructor: return ScriptElementKind.constructorImplementationElement;
            case SyntaxKind.TypeParameter: return ScriptElementKind.typeParameterElement;
            case SyntaxKind.EnumMember: return ScriptElementKind.enumMemberElement;
            case SyntaxKind.Parameter: return hasModifier(node, ModifierFlags.ParameterPropertyModifier) ? ScriptElementKind.memberVariableElement : ScriptElementKind.parameterElement;
            case SyntaxKind.ImportEqualsDeclaration:
            case SyntaxKind.ImportSpecifier:
            case SyntaxKind.ExportSpecifier:
            case SyntaxKind.NamespaceImport:
            case SyntaxKind.NamespaceExport:
                return ScriptElementKind.alias;
            case SyntaxKind.BinaryExpression:
                const kind = getAssignmentDeclarationKind(node as BinaryExpression);
                const { right } = node as BinaryExpression;
                switch (kind) {
                    case AssignmentDeclarationKind.ObjectDefinePropertyValue:
                    case AssignmentDeclarationKind.ObjectDefinePropertyExports:
                    case AssignmentDeclarationKind.ObjectDefinePrototypeProperty:
                    case AssignmentDeclarationKind.None:
                        return ScriptElementKind.unknown;
                    case AssignmentDeclarationKind.ExportsProperty:
                    case AssignmentDeclarationKind.ModuleExports:
                        const rightKind = getNodeKind(right);
                        return rightKind === ScriptElementKind.unknown ? ScriptElementKind.constElement : rightKind;
                    case AssignmentDeclarationKind.PrototypeProperty:
                        return isFunctionExpression(right) ? ScriptElementKind.memberFunctionElement : ScriptElementKind.memberVariableElement;
                    case AssignmentDeclarationKind.ThisProperty:
                        return ScriptElementKind.memberVariableElement; // property
                    case AssignmentDeclarationKind.Property:
                        // static method / property
                        return isFunctionExpression(right) ? ScriptElementKind.memberFunctionElement : ScriptElementKind.memberVariableElement;
                    case AssignmentDeclarationKind.Prototype:
                        return ScriptElementKind.localClassElement;
                    default: {
                        assertType<never>(kind);
                        return ScriptElementKind.unknown;
                    }
                }
            case SyntaxKind.Identifier:
                return isImportClause(node.parent) ? ScriptElementKind.alias : ScriptElementKind.unknown;
            case SyntaxKind.ExportAssignment:
                const scriptKind = getNodeKind((node as ExportAssignment).expression);
                // If the expression didn't come back with something (like it does for an identifiers)
                return scriptKind === ScriptElementKind.unknown ? ScriptElementKind.constElement : scriptKind;
            default:
                return ScriptElementKind.unknown;
        }

        function getKindOfVariableDeclaration(v: VariableDeclaration): ScriptElementKind {
            return isVarConst(v)
                ? ScriptElementKind.constElement
                : isLet(v)
                    ? ScriptElementKind.letElement
                    : ScriptElementKind.variableElement;
        }
    }

    export function isThis(node: Node): boolean {
        switch (node.kind) {
            case SyntaxKind.ThisKeyword:
                // case SyntaxKind.ThisType: TODO: GH#9267
                return true;
            case SyntaxKind.Identifier:
                // 'this' as a parameter
                return identifierIsThisKeyword(node as Identifier) && node.parent.kind === SyntaxKind.Parameter;
            default:
                return false;
        }
    }

    // Matches the beginning of a triple slash directive
    const tripleSlashDirectivePrefixRegex = /^\/\/\/\s*</;

    export interface ListItemInfo {
        listItemIndex: number;
        list: Node;
    }

    export function getLineStartPositionForPosition(position: number, sourceFile: SourceFileLike): number {
        const lineStarts = getLineStarts(sourceFile);
        const line = sourceFile.getLineAndCharacterOfPosition(position).line;
        return lineStarts[line];
    }

    export function rangeContainsRange(r1: TextRange, r2: TextRange): boolean {
        return startEndContainsRange(r1.pos, r1.end, r2);
    }

    export function rangeContainsRangeExclusive(r1: TextRange, r2: TextRange): boolean {
        return rangeContainsPositionExclusive(r1, r2.pos) && rangeContainsPositionExclusive(r1, r2.end);
    }

    export function rangeContainsPosition(r: TextRange, pos: number): boolean {
        return r.pos <= pos && pos <= r.end;
    }

    export function rangeContainsPositionExclusive(r: TextRange, pos: number) {
        return r.pos < pos && pos < r.end;
    }

    export function startEndContainsRange(start: number, end: number, range: TextRange): boolean {
        return start <= range.pos && end >= range.end;
    }

    export function rangeContainsStartEnd(range: TextRange, start: number, end: number): boolean {
        return range.pos <= start && range.end >= end;
    }

    export function rangeOverlapsWithStartEnd(r1: TextRange, start: number, end: number) {
        return startEndOverlapsWithStartEnd(r1.pos, r1.end, start, end);
    }

    export function nodeOverlapsWithStartEnd(node: Node, sourceFile: SourceFile, start: number, end: number) {
        return startEndOverlapsWithStartEnd(node.getStart(sourceFile), node.end, start, end);
    }

    export function startEndOverlapsWithStartEnd(start1: number, end1: number, start2: number, end2: number) {
        const start = Math.max(start1, start2);
        const end = Math.min(end1, end2);
        return start < end;
    }

    /**
     * Assumes `candidate.start <= position` holds.
     */
    export function positionBelongsToNode(candidate: Node, position: number, sourceFile: SourceFile): boolean {
        Debug.assert(candidate.pos <= position);
        return position < candidate.end || !isCompletedNode(candidate, sourceFile);
    }

    function isCompletedNode(n: Node | undefined, sourceFile: SourceFile): boolean {
        if (n === undefined || nodeIsMissing(n)) {
            return false;
        }

        switch (n.kind) {
            case SyntaxKind.ClassDeclaration:
            case SyntaxKind.InterfaceDeclaration:
            case SyntaxKind.EnumDeclaration:
            case SyntaxKind.ObjectLiteralExpression:
            case SyntaxKind.ObjectBindingPattern:
            case SyntaxKind.TypeLiteral:
            case SyntaxKind.Block:
            case SyntaxKind.ModuleBlock:
            case SyntaxKind.CaseBlock:
            case SyntaxKind.NamedImports:
            case SyntaxKind.NamedExports:
                return nodeEndsWith(n, SyntaxKind.CloseBraceToken, sourceFile);
            case SyntaxKind.CatchClause:
                return isCompletedNode((<CatchClause>n).block, sourceFile);
            case SyntaxKind.NewExpression:
                if (!(<NewExpression>n).arguments) {
                    return true;
                }
                // falls through

            case SyntaxKind.CallExpression:
            case SyntaxKind.ParenthesizedExpression:
            case SyntaxKind.ParenthesizedType:
                return nodeEndsWith(n, SyntaxKind.CloseParenToken, sourceFile);

            case SyntaxKind.FunctionType:
            case SyntaxKind.ConstructorType:
                return isCompletedNode((<SignatureDeclaration>n).type, sourceFile);

            case SyntaxKind.Constructor:
            case SyntaxKind.GetAccessor:
            case SyntaxKind.SetAccessor:
            case SyntaxKind.FunctionDeclaration:
            case SyntaxKind.FunctionExpression:
            case SyntaxKind.MethodDeclaration:
            case SyntaxKind.MethodSignature:
            case SyntaxKind.ConstructSignature:
            case SyntaxKind.CallSignature:
            case SyntaxKind.ArrowFunction:
                if ((<FunctionLikeDeclaration>n).body) {
                    return isCompletedNode((<FunctionLikeDeclaration>n).body, sourceFile);
                }

                if ((<FunctionLikeDeclaration>n).type) {
                    return isCompletedNode((<FunctionLikeDeclaration>n).type, sourceFile);
                }

                // Even though type parameters can be unclosed, we can get away with
                // having at least a closing paren.
                return hasChildOfKind(n, SyntaxKind.CloseParenToken, sourceFile);

            case SyntaxKind.ModuleDeclaration:
                return !!(<ModuleDeclaration>n).body && isCompletedNode((<ModuleDeclaration>n).body, sourceFile);

            case SyntaxKind.IfStatement:
                if ((<IfStatement>n).elseStatement) {
                    return isCompletedNode((<IfStatement>n).elseStatement, sourceFile);
                }
                return isCompletedNode((<IfStatement>n).thenStatement, sourceFile);

            case SyntaxKind.ExpressionStatement:
                return isCompletedNode((<ExpressionStatement>n).expression, sourceFile) ||
                    hasChildOfKind(n, SyntaxKind.SemicolonToken, sourceFile);

            case SyntaxKind.ArrayLiteralExpression:
            case SyntaxKind.ArrayBindingPattern:
            case SyntaxKind.ElementAccessExpression:
            case SyntaxKind.ComputedPropertyName:
            case SyntaxKind.TupleType:
                return nodeEndsWith(n, SyntaxKind.CloseBracketToken, sourceFile);

            case SyntaxKind.IndexSignature:
                if ((<IndexSignatureDeclaration>n).type) {
                    return isCompletedNode((<IndexSignatureDeclaration>n).type, sourceFile);
                }

                return hasChildOfKind(n, SyntaxKind.CloseBracketToken, sourceFile);

            case SyntaxKind.CaseClause:
            case SyntaxKind.DefaultClause:
                // there is no such thing as terminator token for CaseClause/DefaultClause so for simplicity always consider them non-completed
                return false;

            case SyntaxKind.ForStatement:
            case SyntaxKind.ForInStatement:
            case SyntaxKind.ForOfStatement:
            case SyntaxKind.WhileStatement:
                return isCompletedNode((<IterationStatement>n).statement, sourceFile);
            case SyntaxKind.DoStatement:
                // rough approximation: if DoStatement has While keyword - then if node is completed is checking the presence of ')';
                return hasChildOfKind(n, SyntaxKind.WhileKeyword, sourceFile)
                    ? nodeEndsWith(n, SyntaxKind.CloseParenToken, sourceFile)
                    : isCompletedNode((<DoStatement>n).statement, sourceFile);

            case SyntaxKind.TypeQuery:
                return isCompletedNode((<TypeQueryNode>n).exprName, sourceFile);

            case SyntaxKind.TypeOfExpression:
            case SyntaxKind.DeleteExpression:
            case SyntaxKind.VoidExpression:
            case SyntaxKind.YieldExpression:
            case SyntaxKind.SpreadElement:
                const unaryWordExpression = n as (TypeOfExpression | DeleteExpression | VoidExpression | YieldExpression | SpreadElement);
                return isCompletedNode(unaryWordExpression.expression, sourceFile);

            case SyntaxKind.TaggedTemplateExpression:
                return isCompletedNode((<TaggedTemplateExpression>n).template, sourceFile);
            case SyntaxKind.TemplateExpression:
                const lastSpan = lastOrUndefined((<TemplateExpression>n).templateSpans);
                return isCompletedNode(lastSpan, sourceFile);
            case SyntaxKind.TemplateSpan:
                return nodeIsPresent((<TemplateSpan>n).literal);

            case SyntaxKind.ExportDeclaration:
            case SyntaxKind.ImportDeclaration:
                return nodeIsPresent((<ExportDeclaration | ImportDeclaration>n).moduleSpecifier);

            case SyntaxKind.PrefixUnaryExpression:
                return isCompletedNode((<PrefixUnaryExpression>n).operand, sourceFile);
            case SyntaxKind.BinaryExpression:
                return isCompletedNode((<BinaryExpression>n).right, sourceFile);
            case SyntaxKind.ConditionalExpression:
                return isCompletedNode((<ConditionalExpression>n).whenFalse, sourceFile);

            default:
                return true;
        }
    }

    /*
     * Checks if node ends with 'expectedLastToken'.
     * If child at position 'length - 1' is 'SemicolonToken' it is skipped and 'expectedLastToken' is compared with child at position 'length - 2'.
     */
    function nodeEndsWith(n: Node, expectedLastToken: SyntaxKind, sourceFile: SourceFile): boolean {
        const children = n.getChildren(sourceFile);
        if (children.length) {
            const lastChild = last(children);
            if (lastChild.kind === expectedLastToken) {
                return true;
            }
            else if (lastChild.kind === SyntaxKind.SemicolonToken && children.length !== 1) {
                return children[children.length - 2].kind === expectedLastToken;
            }
        }
        return false;
    }

    export function findListItemInfo(node: Node): ListItemInfo | undefined {
        const list = findContainingList(node);

        // It is possible at this point for syntaxList to be undefined, either if
        // node.parent had no list child, or if none of its list children contained
        // the span of node. If this happens, return undefined. The caller should
        // handle this case.
        if (!list) {
            return undefined;
        }

        const children = list.getChildren();
        const listItemIndex = indexOfNode(children, node);

        return {
            listItemIndex,
            list
        };
    }

    export function hasChildOfKind(n: Node, kind: SyntaxKind, sourceFile: SourceFile): boolean {
        return !!findChildOfKind(n, kind, sourceFile);
    }

    export function findChildOfKind<T extends Node>(n: Node, kind: T["kind"], sourceFile: SourceFileLike): T | undefined {
        return find(n.getChildren(sourceFile), (c): c is T => c.kind === kind);
    }

    export function findContainingList(node: Node): SyntaxList | undefined {
        // The node might be a list element (nonsynthetic) or a comma (synthetic). Either way, it will
        // be parented by the container of the SyntaxList, not the SyntaxList itself.
        // In order to find the list item index, we first need to locate SyntaxList itself and then search
        // for the position of the relevant node (or comma).
        const syntaxList = find(node.parent.getChildren(), (c): c is SyntaxList => isSyntaxList(c) && rangeContainsRange(c, node));
        // Either we didn't find an appropriate list, or the list must contain us.
        Debug.assert(!syntaxList || contains(syntaxList.getChildren(), node));
        return syntaxList;
    }

    /**
     * Gets the token whose text has range [start, end) and
     * position >= start and (position < end or (position === end && token is literal or keyword or identifier))
     */
    export function getTouchingPropertyName(sourceFile: SourceFile, position: number): Node {
        return getTouchingToken(sourceFile, position, n => isPropertyNameLiteral(n) || isKeyword(n.kind) || isPrivateIdentifier(n));
    }

    /**
     * Returns the token if position is in [start, end).
     * If position === end, returns the preceding token if includeItemAtEndPosition(previousToken) === true
     */
    export function getTouchingToken(sourceFile: SourceFile, position: number, includePrecedingTokenAtEndPosition?: (n: Node) => boolean): Node {
        return getTokenAtPositionWorker(sourceFile, position, /*allowPositionInLeadingTrivia*/ false, includePrecedingTokenAtEndPosition, /*includeEndPosition*/ false);
    }

    /** Returns a token if position is in [start-of-leading-trivia, end) */
    export function getTokenAtPosition(sourceFile: SourceFile, position: number): Node {
        return getTokenAtPositionWorker(sourceFile, position, /*allowPositionInLeadingTrivia*/ true, /*includePrecedingTokenAtEndPosition*/ undefined, /*includeEndPosition*/ false);
    }

    /** Get the token whose text contains the position */
    function getTokenAtPositionWorker(sourceFile: SourceFile, position: number, allowPositionInLeadingTrivia: boolean, includePrecedingTokenAtEndPosition: ((n: Node) => boolean) | undefined, includeEndPosition: boolean): Node {
        let current: Node = sourceFile;
        outer: while (true) {
            // find the child that contains 'position'
            for (const child of current.getChildren(sourceFile)) {
                const start = allowPositionInLeadingTrivia ? child.getFullStart() : child.getStart(sourceFile, /*includeJsDoc*/ true);
                if (start > position) {
                    // If this child begins after position, then all subsequent children will as well.
                    break;
                }

                const end = child.getEnd();
                if (position < end || (position === end && (child.kind === SyntaxKind.EndOfFileToken || includeEndPosition))) {
                    current = child;
                    continue outer;
                }
                else if (includePrecedingTokenAtEndPosition && end === position) {
                    const previousToken = findPrecedingToken(position, sourceFile, child);
                    if (previousToken && includePrecedingTokenAtEndPosition(previousToken)) {
                        return previousToken;
                    }
                }
            }

            return current;
        }
    }

    /**
     * The token on the left of the position is the token that strictly includes the position
     * or sits to the left of the cursor if it is on a boundary. For example
     *
     *   fo|o               -> will return foo
     *   foo <comment> |bar -> will return foo
     *
     */
    export function findTokenOnLeftOfPosition(file: SourceFile, position: number): Node | undefined {
        // Ideally, getTokenAtPosition should return a token. However, it is currently
        // broken, so we do a check to make sure the result was indeed a token.
        const tokenAtPosition = getTokenAtPosition(file, position);
        if (isToken(tokenAtPosition) && position > tokenAtPosition.getStart(file) && position < tokenAtPosition.getEnd()) {
            return tokenAtPosition;
        }

        return findPrecedingToken(position, file);
    }

    export function findNextToken(previousToken: Node, parent: Node, sourceFile: SourceFileLike): Node | undefined {
        return find(parent);

        function find(n: Node): Node | undefined {
            if (isToken(n) && n.pos === previousToken.end) {
                // this is token that starts at the end of previous token - return it
                return n;
            }
            return firstDefined(n.getChildren(sourceFile), child => {
                const shouldDiveInChildNode =
                    // previous token is enclosed somewhere in the child
                    (child.pos <= previousToken.pos && child.end > previousToken.end) ||
                    // previous token ends exactly at the beginning of child
                    (child.pos === previousToken.end);
                return shouldDiveInChildNode && nodeHasTokens(child, sourceFile) ? find(child) : undefined;
            });
        }
    }

    /**
     * Finds the rightmost token satisfying `token.end <= position`,
     * excluding `JsxText` tokens containing only whitespace.
     */
    export function findPrecedingToken(position: number, sourceFile: SourceFile, startNode?: Node, excludeJsdoc?: boolean): Node | undefined {
        const result = find(startNode || sourceFile);
        Debug.assert(!(result && isWhiteSpaceOnlyJsxText(result)));
        return result;

        function find(n: Node): Node | undefined {
            if (isNonWhitespaceToken(n) && n.kind !== SyntaxKind.EndOfFileToken) {
                return n;
            }

            const children = n.getChildren(sourceFile);
            for (let i = 0; i < children.length; i++) {
                const child = children[i];
                // Note that the span of a node's tokens is [node.getStart(...), node.end).
                // Given that `position < child.end` and child has constituent tokens, we distinguish these cases:
                // 1) `position` precedes `child`'s tokens or `child` has no tokens (ie: in a comment or whitespace preceding `child`):
                // we need to find the last token in a previous child.
                // 2) `position` is within the same span: we recurse on `child`.
                if (position < child.end) {
                    const start = child.getStart(sourceFile, /*includeJsDoc*/ !excludeJsdoc);
                    const lookInPreviousChild =
                        (start >= position) || // cursor in the leading trivia
                        !nodeHasTokens(child, sourceFile) ||
                        isWhiteSpaceOnlyJsxText(child);

                    if (lookInPreviousChild) {
                        // actual start of the node is past the position - previous token should be at the end of previous child
                        const candidate = findRightmostChildNodeWithTokens(children, /*exclusiveStartPosition*/ i, sourceFile);
                        return candidate && findRightmostToken(candidate, sourceFile);
                    }
                    else {
                        // candidate should be in this node
                        return find(child);
                    }
                }
            }

            Debug.assert(startNode !== undefined || n.kind === SyntaxKind.SourceFile || n.kind === SyntaxKind.EndOfFileToken || isJSDocCommentContainingNode(n));

            // Here we know that none of child token nodes embrace the position,
            // the only known case is when position is at the end of the file.
            // Try to find the rightmost token in the file without filtering.
            // Namely we are skipping the check: 'position < node.end'
            const candidate = findRightmostChildNodeWithTokens(children, /*exclusiveStartPosition*/ children.length, sourceFile);
            return candidate && findRightmostToken(candidate, sourceFile);
        }
    }

    function isNonWhitespaceToken(n: Node): boolean {
        return isToken(n) && !isWhiteSpaceOnlyJsxText(n);
    }

    function findRightmostToken(n: Node, sourceFile: SourceFile): Node | undefined {
        if (isNonWhitespaceToken(n)) {
            return n;
        }

        const children = n.getChildren(sourceFile);
        const candidate = findRightmostChildNodeWithTokens(children, /*exclusiveStartPosition*/ children.length, sourceFile);
        return candidate && findRightmostToken(candidate, sourceFile);
    }

    /**
     * Finds the rightmost child to the left of `children[exclusiveStartPosition]` which is a non-all-whitespace token or has constituent tokens.
     */
    function findRightmostChildNodeWithTokens(children: Node[], exclusiveStartPosition: number, sourceFile: SourceFile): Node | undefined {
        for (let i = exclusiveStartPosition - 1; i >= 0; i--) {
            const child = children[i];

            if (isWhiteSpaceOnlyJsxText(child)) {
                Debug.assert(i > 0, "`JsxText` tokens should not be the first child of `JsxElement | JsxSelfClosingElement`");
            }
            else if (nodeHasTokens(children[i], sourceFile)) {
                return children[i];
            }
        }
    }

    export function isInString(sourceFile: SourceFile, position: number, previousToken = findPrecedingToken(position, sourceFile)): boolean {
        if (previousToken && isStringTextContainingNode(previousToken)) {
            const start = previousToken.getStart(sourceFile);
            const end = previousToken.getEnd();

            // To be "in" one of these literals, the position has to be:
            //   1. entirely within the token text.
            //   2. at the end position of an unterminated token.
            //   3. at the end of a regular expression (due to trailing flags like '/foo/g').
            if (start < position && position < end) {
                return true;
            }

            if (position === end) {
                return !!(<LiteralExpression>previousToken).isUnterminated;
            }
        }

        return false;
    }

    /**
     * returns true if the position is in between the open and close elements of an JSX expression.
     */
    export function isInsideJsxElementOrAttribute(sourceFile: SourceFile, position: number) {
        const token = getTokenAtPosition(sourceFile, position);

        if (!token) {
            return false;
        }

        if (token.kind === SyntaxKind.JsxText) {
            return true;
        }

        // <div>Hello |</div>
        if (token.kind === SyntaxKind.LessThanToken && token.parent.kind === SyntaxKind.JsxText) {
            return true;
        }

        // <div> { | </div> or <div a={| </div>
        if (token.kind === SyntaxKind.LessThanToken && token.parent.kind === SyntaxKind.JsxExpression) {
            return true;
        }

        // <div> {
        // |
        // } < /div>
        if (token && token.kind === SyntaxKind.CloseBraceToken && token.parent.kind === SyntaxKind.JsxExpression) {
            return true;
        }

        // <div>|</div>
        if (token.kind === SyntaxKind.LessThanToken && token.parent.kind === SyntaxKind.JsxClosingElement) {
            return true;
        }

        return false;
    }

    function isWhiteSpaceOnlyJsxText(node: Node): boolean {
        return isJsxText(node) && node.containsOnlyTriviaWhiteSpaces;
    }

    export function isInTemplateString(sourceFile: SourceFile, position: number) {
        const token = getTokenAtPosition(sourceFile, position);
        return isTemplateLiteralKind(token.kind) && position > token.getStart(sourceFile);
    }

    export function isInJSXText(sourceFile: SourceFile, position: number) {
        const token = getTokenAtPosition(sourceFile, position);
        if (isJsxText(token)) {
            return true;
        }
        if (token.kind === SyntaxKind.OpenBraceToken && isJsxExpression(token.parent) && isJsxElement(token.parent.parent)) {
            return true;
        }
        if (token.kind === SyntaxKind.LessThanToken && isJsxOpeningLikeElement(token.parent) && isJsxElement(token.parent.parent)) {
            return true;
        }
        return false;
    }

    export function findPrecedingMatchingToken(token: Node, matchingTokenKind: SyntaxKind, sourceFile: SourceFile) {
        const tokenKind = token.kind;
        let remainingMatchingTokens = 0;
        while (true) {
            const preceding = findPrecedingToken(token.getFullStart(), sourceFile);
            if (!preceding) {
                return undefined;
            }
            token = preceding;

            if (token.kind === matchingTokenKind) {
                if (remainingMatchingTokens === 0) {
                    return token;
                }

                remainingMatchingTokens--;
            }
            else if (token.kind === tokenKind) {
                remainingMatchingTokens++;
            }
        }
    }

    export function removeOptionality(type: Type, isOptionalExpression: boolean, isOptionalChain: boolean) {
        return isOptionalExpression ? type.getNonNullableType() :
            isOptionalChain ? type.getNonOptionalType() :
            type;
    }

    export function isPossiblyTypeArgumentPosition(token: Node, sourceFile: SourceFile, checker: TypeChecker): boolean {
        const info = getPossibleTypeArgumentsInfo(token, sourceFile);
        return info !== undefined && (isPartOfTypeNode(info.called) ||
            getPossibleGenericSignatures(info.called, info.nTypeArguments, checker).length !== 0 ||
            isPossiblyTypeArgumentPosition(info.called, sourceFile, checker));
    }

    export function getPossibleGenericSignatures(called: Expression, typeArgumentCount: number, checker: TypeChecker): readonly Signature[] {
        let type = checker.getTypeAtLocation(called);
        if (isOptionalChain(called.parent)) {
            type = removeOptionality(type, !!called.parent.questionDotToken, /*isOptionalChain*/ true);
        }

        const signatures = isNewExpression(called.parent) ? type.getConstructSignatures() : type.getCallSignatures();
        return signatures.filter(candidate => !!candidate.typeParameters && candidate.typeParameters.length >= typeArgumentCount);
    }

    export interface PossibleTypeArgumentInfo {
        readonly called: Identifier;
        readonly nTypeArguments: number;
    }

    export interface PossibleProgramFileInfo {
        ProgramFiles?: string[];
    }

    // Get info for an expression like `f <` that may be the start of type arguments.
    export function getPossibleTypeArgumentsInfo(tokenIn: Node, sourceFile: SourceFile): PossibleTypeArgumentInfo | undefined {
        let token: Node | undefined = tokenIn;
        // This function determines if the node could be type argument position
        // Since during editing, when type argument list is not complete,
        // the tree could be of any shape depending on the tokens parsed before current node,
        // scanning of the previous identifier followed by "<" before current node would give us better result
        // Note that we also balance out the already provided type arguments, arrays, object literals while doing so
        let remainingLessThanTokens = 0;
        let nTypeArguments = 0;
        while (token) {
            switch (token.kind) {
                case SyntaxKind.LessThanToken:
                    // Found the beginning of the generic argument expression
                    token = findPrecedingToken(token.getFullStart(), sourceFile);
                    if (token && token.kind === SyntaxKind.QuestionDotToken) {
                        token = findPrecedingToken(token.getFullStart(), sourceFile);
                    }
                    if (!token || !isIdentifier(token)) return undefined;
                    if (!remainingLessThanTokens) {
                        return isDeclarationName(token) ? undefined : { called: token, nTypeArguments };
                    }
                    remainingLessThanTokens--;
                    break;

                case SyntaxKind.GreaterThanGreaterThanGreaterThanToken:
                    remainingLessThanTokens = + 3;
                    break;

                case SyntaxKind.GreaterThanGreaterThanToken:
                    remainingLessThanTokens = + 2;
                    break;

                case SyntaxKind.GreaterThanToken:
                    remainingLessThanTokens++;
                    break;

                case SyntaxKind.CloseBraceToken:
                    // This can be object type, skip until we find the matching open brace token
                    // Skip until the matching open brace token
                    token = findPrecedingMatchingToken(token, SyntaxKind.OpenBraceToken, sourceFile);
                    if (!token) return undefined;
                    break;

                case SyntaxKind.CloseParenToken:
                    // This can be object type, skip until we find the matching open brace token
                    // Skip until the matching open brace token
                    token = findPrecedingMatchingToken(token, SyntaxKind.OpenParenToken, sourceFile);
                    if (!token) return undefined;
                    break;

                case SyntaxKind.CloseBracketToken:
                    // This can be object type, skip until we find the matching open brace token
                    // Skip until the matching open brace token
                    token = findPrecedingMatchingToken(token, SyntaxKind.OpenBracketToken, sourceFile);
                    if (!token) return undefined;
                    break;

                // Valid tokens in a type name. Skip.
                case SyntaxKind.CommaToken:
                    nTypeArguments++;
                    break;

                case SyntaxKind.EqualsGreaterThanToken:
                    // falls through

                case SyntaxKind.Identifier:
                case SyntaxKind.StringLiteral:
                case SyntaxKind.NumericLiteral:
                case SyntaxKind.BigIntLiteral:
                case SyntaxKind.TrueKeyword:
                case SyntaxKind.FalseKeyword:
                    // falls through

                case SyntaxKind.TypeOfKeyword:
                case SyntaxKind.ExtendsKeyword:
                case SyntaxKind.KeyOfKeyword:
                case SyntaxKind.DotToken:
                case SyntaxKind.BarToken:
                case SyntaxKind.QuestionToken:
                case SyntaxKind.ColonToken:
                    break;

                default:
                    if (isTypeNode(token)) {
                        break;
                    }

                    // Invalid token in type
                    return undefined;
            }

            token = findPrecedingToken(token.getFullStart(), sourceFile);
        }

        return undefined;
    }

    /**
     * Returns true if the cursor at position in sourceFile is within a comment.
     *
     * @param tokenAtPosition Must equal `getTokenAtPosition(sourceFile, position)
     * @param predicate Additional predicate to test on the comment range.
     */
    export function isInComment(sourceFile: SourceFile, position: number, tokenAtPosition?: Node): CommentRange | undefined {
        return formatting.getRangeOfEnclosingComment(sourceFile, position, /*precedingToken*/ undefined, tokenAtPosition);
    }

    export function hasDocComment(sourceFile: SourceFile, position: number): boolean {
        const token = getTokenAtPosition(sourceFile, position);
        return !!findAncestor(token, isJSDoc);
    }

    function nodeHasTokens(n: Node, sourceFile: SourceFileLike): boolean {
        // If we have a token or node that has a non-zero width, it must have tokens.
        // Note: getWidth() does not take trivia into account.
        return n.kind === SyntaxKind.EndOfFileToken ? !!(n as EndOfFileToken).jsDoc : n.getWidth(sourceFile) !== 0;
    }

    export function getNodeModifiers(node: Node): string {
        const flags = isDeclaration(node) ? getCombinedModifierFlags(node) : ModifierFlags.None;
        const result: string[] = [];

        if (flags & ModifierFlags.Private) result.push(ScriptElementKindModifier.privateMemberModifier);
        if (flags & ModifierFlags.Protected) result.push(ScriptElementKindModifier.protectedMemberModifier);
        if (flags & ModifierFlags.Public) result.push(ScriptElementKindModifier.publicMemberModifier);
        if (flags & ModifierFlags.Static) result.push(ScriptElementKindModifier.staticModifier);
        if (flags & ModifierFlags.Abstract) result.push(ScriptElementKindModifier.abstractModifier);
        if (flags & ModifierFlags.Export) result.push(ScriptElementKindModifier.exportedModifier);
        if (node.flags & NodeFlags.Ambient) result.push(ScriptElementKindModifier.ambientModifier);
        if (node.kind === SyntaxKind.ExportAssignment) result.push(ScriptElementKindModifier.exportedModifier);

        return result.length > 0 ? result.join(",") : ScriptElementKindModifier.none;
    }

    export function getTypeArgumentOrTypeParameterList(node: Node): NodeArray<Node> | undefined {
        if (node.kind === SyntaxKind.TypeReference || node.kind === SyntaxKind.CallExpression) {
            return (<CallExpression>node).typeArguments;
        }

        if (isFunctionLike(node) || node.kind === SyntaxKind.ClassDeclaration || node.kind === SyntaxKind.InterfaceDeclaration) {
            return (<FunctionLikeDeclaration>node).typeParameters;
        }

        return undefined;
    }

    export function isComment(kind: SyntaxKind): boolean {
        return kind === SyntaxKind.SingleLineCommentTrivia || kind === SyntaxKind.MultiLineCommentTrivia;
    }

    export function isStringOrRegularExpressionOrTemplateLiteral(kind: SyntaxKind): boolean {
        if (kind === SyntaxKind.StringLiteral
            || kind === SyntaxKind.RegularExpressionLiteral
            || isTemplateLiteralKind(kind)) {
            return true;
        }
        return false;
    }

    export function isPunctuation(kind: SyntaxKind): boolean {
        return SyntaxKind.FirstPunctuation <= kind && kind <= SyntaxKind.LastPunctuation;
    }

    export function isInsideTemplateLiteral(node: TemplateLiteralToken, position: number, sourceFile: SourceFile): boolean {
        return isTemplateLiteralKind(node.kind)
            && (node.getStart(sourceFile) < position && position < node.end) || (!!node.isUnterminated && position === node.end);
    }

    export function isAccessibilityModifier(kind: SyntaxKind) {
        switch (kind) {
            case SyntaxKind.PublicKeyword:
            case SyntaxKind.PrivateKeyword:
            case SyntaxKind.ProtectedKeyword:
                return true;
        }

        return false;
    }

    export function cloneCompilerOptions(options: CompilerOptions): CompilerOptions {
        const result = clone(options);
        setConfigFileInOptions(result, options && options.configFile);
        return result;
    }

    export function isArrayLiteralOrObjectLiteralDestructuringPattern(node: Node) {
        if (node.kind === SyntaxKind.ArrayLiteralExpression ||
            node.kind === SyntaxKind.ObjectLiteralExpression) {
            // [a,b,c] from:
            // [a, b, c] = someExpression;
            if (node.parent.kind === SyntaxKind.BinaryExpression &&
                (<BinaryExpression>node.parent).left === node &&
                (<BinaryExpression>node.parent).operatorToken.kind === SyntaxKind.EqualsToken) {
                return true;
            }

            // [a, b, c] from:
            // for([a, b, c] of expression)
            if (node.parent.kind === SyntaxKind.ForOfStatement &&
                (<ForOfStatement>node.parent).initializer === node) {
                return true;
            }

            // [a, b, c] of
            // [x, [a, b, c] ] = someExpression
            // or
            // {x, a: {a, b, c} } = someExpression
            if (isArrayLiteralOrObjectLiteralDestructuringPattern(node.parent.kind === SyntaxKind.PropertyAssignment ? node.parent.parent : node.parent)) {
                return true;
            }
        }

        return false;
    }

    export function isInReferenceComment(sourceFile: SourceFile, position: number): boolean {
        return isInReferenceCommentWorker(sourceFile, position, /*shouldBeReference*/ true);
    }

    export function isInNonReferenceComment(sourceFile: SourceFile, position: number): boolean {
        return isInReferenceCommentWorker(sourceFile, position, /*shouldBeReference*/ false);
    }

    function isInReferenceCommentWorker(sourceFile: SourceFile, position: number, shouldBeReference: boolean): boolean {
        const range = isInComment(sourceFile, position, /*tokenAtPosition*/ undefined);
        return !!range && shouldBeReference === tripleSlashDirectivePrefixRegex.test(sourceFile.text.substring(range.pos, range.end));
    }

    export function createTextSpanFromNode(node: Node, sourceFile?: SourceFile, endNode?: Node): TextSpan {
        return createTextSpanFromBounds(node.getStart(sourceFile), (endNode || node).getEnd());
    }

    export function createTextRangeFromNode(node: Node, sourceFile: SourceFile): TextRange {
        return createRange(node.getStart(sourceFile), node.end);
    }

    export function createTextSpanFromRange(range: TextRange): TextSpan {
        return createTextSpanFromBounds(range.pos, range.end);
    }

    export function createTextRangeFromSpan(span: TextSpan): TextRange {
        return createRange(span.start, span.start + span.length);
    }

    export function createTextChangeFromStartLength(start: number, length: number, newText: string): TextChange {
        return createTextChange(createTextSpan(start, length), newText);
    }

    export function createTextChange(span: TextSpan, newText: string): TextChange {
        return { span, newText };
    }

    export const typeKeywords: readonly SyntaxKind[] = [
        SyntaxKind.AnyKeyword,
        SyntaxKind.AssertsKeyword,
        SyntaxKind.BigIntKeyword,
        SyntaxKind.BooleanKeyword,
        SyntaxKind.FalseKeyword,
        SyntaxKind.KeyOfKeyword,
        SyntaxKind.NeverKeyword,
        SyntaxKind.NullKeyword,
        SyntaxKind.NumberKeyword,
        SyntaxKind.ObjectKeyword,
        SyntaxKind.ReadonlyKeyword,
        SyntaxKind.StringKeyword,
        SyntaxKind.SymbolKeyword,
        SyntaxKind.TrueKeyword,
        SyntaxKind.VoidKeyword,
        SyntaxKind.UndefinedKeyword,
        SyntaxKind.UniqueKeyword,
        SyntaxKind.UnknownKeyword,
    ];

    export function isTypeKeyword(kind: SyntaxKind): boolean {
        return contains(typeKeywords, kind);
    }

    /** True if the symbol is for an external module, as opposed to a namespace. */
    export function isExternalModuleSymbol(moduleSymbol: Symbol): boolean {
        return !!(moduleSymbol.flags & SymbolFlags.Module) && moduleSymbol.name.charCodeAt(0) === CharacterCodes.doubleQuote;
    }

    /** Returns `true` the first time it encounters a node and `false` afterwards. */
    export type NodeSeenTracker<T = Node> = (node: T) => boolean;
    export function nodeSeenTracker<T extends Node>(): NodeSeenTracker<T> {
        const seen: true[] = [];
        return node => {
            const id = getNodeId(node);
            return !seen[id] && (seen[id] = true);
        };
    }

    export function getSnapshotText(snap: IScriptSnapshot): string {
        return snap.getText(0, snap.getLength());
    }

    export function repeatString(str: string, count: number): string {
        let result = "";
        for (let i = 0; i < count; i++) {
            result += str;
        }
        return result;
    }

    export function skipConstraint(type: Type): Type {
        return type.isTypeParameter() ? type.getConstraint() || type : type;
    }

    export function getNameFromPropertyName(name: PropertyName): string | undefined {
        return name.kind === SyntaxKind.ComputedPropertyName
            // treat computed property names where expression is string/numeric literal as just string/numeric literal
            ? isStringOrNumericLiteralLike(name.expression) ? name.expression.text : undefined
            : isPrivateIdentifier(name) ? idText(name) : getTextOfIdentifierOrLiteral(name);
    }

    export function programContainsEs6Modules(program: Program): boolean {
        return program.getSourceFiles().some(s => !s.isDeclarationFile && !program.isSourceFileFromExternalLibrary(s) && !!s.externalModuleIndicator);
    }
    export function compilerOptionsIndicateEs6Modules(compilerOptions: CompilerOptions): boolean {
        return !!compilerOptions.module || compilerOptions.target! >= ScriptTarget.ES2015 || !!compilerOptions.noEmit;
    }

    export function hostUsesCaseSensitiveFileNames(host: { useCaseSensitiveFileNames?(): boolean; }): boolean {
        return host.useCaseSensitiveFileNames ? host.useCaseSensitiveFileNames() : false;
    }

    export function hostGetCanonicalFileName(host: { useCaseSensitiveFileNames?(): boolean; }): GetCanonicalFileName {
        return createGetCanonicalFileName(hostUsesCaseSensitiveFileNames(host));
    }

    export function makeImportIfNecessary(defaultImport: Identifier | undefined, namedImports: readonly ImportSpecifier[] | undefined, moduleSpecifier: string, quotePreference: QuotePreference): ImportDeclaration | undefined {
        return defaultImport || namedImports && namedImports.length ? makeImport(defaultImport, namedImports, moduleSpecifier, quotePreference) : undefined;
    }

    export function makeImport(defaultImport: Identifier | undefined, namedImports: readonly ImportSpecifier[] | undefined, moduleSpecifier: string | Expression, quotePreference: QuotePreference, isTypeOnly?: boolean): ImportDeclaration {
        return createImportDeclaration(
            /*decorators*/ undefined,
            /*modifiers*/ undefined,
            defaultImport || namedImports
                ? createImportClause(defaultImport, namedImports && namedImports.length ? createNamedImports(namedImports) : undefined, isTypeOnly)
                : undefined,
            typeof moduleSpecifier === "string" ? makeStringLiteral(moduleSpecifier, quotePreference) : moduleSpecifier);
    }

    export function makeStringLiteral(text: string, quotePreference: QuotePreference): StringLiteral {
        return createLiteral(text, quotePreference === QuotePreference.Single);
    }

    export const enum QuotePreference { Single, Double }

    export function quotePreferenceFromString(str: StringLiteral, sourceFile: SourceFile): QuotePreference {
        return isStringDoubleQuoted(str, sourceFile) ? QuotePreference.Double : QuotePreference.Single;
    }

    export function getQuotePreference(sourceFile: SourceFile, preferences: UserPreferences): QuotePreference {
        if (preferences.quotePreference && preferences.quotePreference !== "auto") {
            return preferences.quotePreference === "single" ? QuotePreference.Single : QuotePreference.Double;
        }
        else {
            const firstModuleSpecifier = sourceFile.imports && find(sourceFile.imports, isStringLiteral);
            return firstModuleSpecifier ? quotePreferenceFromString(firstModuleSpecifier, sourceFile) : QuotePreference.Double;
        }
    }

    export function getQuoteFromPreference(qp: QuotePreference): string {
        switch (qp) {
            case QuotePreference.Single: return "'";
            case QuotePreference.Double: return '"';
            default: return Debug.assertNever(qp);
        }
    }

    export function symbolNameNoDefault(symbol: Symbol): string | undefined {
        const escaped = symbolEscapedNameNoDefault(symbol);
        return escaped === undefined ? undefined : unescapeLeadingUnderscores(escaped);
    }

    export function symbolEscapedNameNoDefault(symbol: Symbol): __String | undefined {
        if (symbol.escapedName !== InternalSymbolName.Default) {
            return symbol.escapedName;
        }

        return firstDefined(symbol.declarations, decl => {
            const name = getNameOfDeclaration(decl);
            return name && name.kind === SyntaxKind.Identifier ? name.escapedText : undefined;
        });
    }

    export type ObjectBindingElementWithoutPropertyName = BindingElement & { name: Identifier };

    export function isObjectBindingElementWithoutPropertyName(bindingElement: Node): bindingElement is ObjectBindingElementWithoutPropertyName {
        return isBindingElement(bindingElement) &&
            isObjectBindingPattern(bindingElement.parent) &&
            isIdentifier(bindingElement.name) &&
            !bindingElement.propertyName;
    }

    export function getPropertySymbolFromBindingElement(checker: TypeChecker, bindingElement: ObjectBindingElementWithoutPropertyName): Symbol | undefined {
        const typeOfPattern = checker.getTypeAtLocation(bindingElement.parent);
        return typeOfPattern && checker.getPropertyOfType(typeOfPattern, bindingElement.name.text);
    }

    /**
     * Find symbol of the given property-name and add the symbol to the given result array
     * @param symbol a symbol to start searching for the given propertyName
     * @param propertyName a name of property to search for
     * @param result an array of symbol of found property symbols
     * @param previousIterationSymbolsCache a cache of symbol from previous iterations of calling this function to prevent infinite revisiting of the same symbol.
     *                                The value of previousIterationSymbol is undefined when the function is first called.
     */
    export function getPropertySymbolsFromBaseTypes<T>(symbol: Symbol, propertyName: string, checker: TypeChecker, cb: (symbol: Symbol) => T | undefined): T | undefined {
        const seen = createMap<true>();
        return recur(symbol);

        function recur(symbol: Symbol): T | undefined {
            // Use `addToSeen` to ensure we don't infinitely recurse in this situation:
            //      interface C extends C {
            //          /*findRef*/propName: string;
            //      }
            if (!(symbol.flags & (SymbolFlags.Class | SymbolFlags.Interface)) || !addToSeen(seen, getSymbolId(symbol))) return;

            return firstDefined(symbol.declarations, declaration => firstDefined(getAllSuperTypeNodes(declaration), typeReference => {
                const type = checker.getTypeAtLocation(typeReference);
                const propertySymbol = type && type.symbol && checker.getPropertyOfType(type, propertyName);
                // Visit the typeReference as well to see if it directly or indirectly uses that property
                return type && propertySymbol && (firstDefined(checker.getRootSymbols(propertySymbol), cb) || recur(type.symbol));
            }));
        }
    }

    export function isMemberSymbolInBaseType(memberSymbol: Symbol, checker: TypeChecker): boolean {
        return getPropertySymbolsFromBaseTypes(memberSymbol.parent!, memberSymbol.name, checker, _ => true) || false;
    }

    export function getParentNodeInSpan(node: Node | undefined, file: SourceFile, span: TextSpan): Node | undefined {
        if (!node) return undefined;

        while (node.parent) {
            if (isSourceFile(node.parent) || !spanContainsNode(span, node.parent, file)) {
                return node;
            }

            node = node.parent;
        }
    }

    function spanContainsNode(span: TextSpan, node: Node, file: SourceFile): boolean {
        return textSpanContainsPosition(span, node.getStart(file)) &&
            node.getEnd() <= textSpanEnd(span);
    }

    export function findModifier(node: Node, kind: Modifier["kind"]): Modifier | undefined {
        return node.modifiers && find(node.modifiers, m => m.kind === kind);
    }

    export function insertImport(changes: textChanges.ChangeTracker, sourceFile: SourceFile, importDecl: Statement): void {
        const lastImportDeclaration = findLast(sourceFile.statements, isAnyImportSyntax);
        if (lastImportDeclaration) {
            changes.insertNodeAfter(sourceFile, lastImportDeclaration, importDecl);
        }
        else {
            changes.insertNodeAtTopOfFile(sourceFile, importDecl, /*blankLineBetween*/ true);
        }
    }

    export function textSpansEqual(a: TextSpan | undefined, b: TextSpan | undefined): boolean {
        return !!a && !!b && a.start === b.start && a.length === b.length;
    }
    export function documentSpansEqual(a: DocumentSpan, b: DocumentSpan): boolean {
        return a.fileName === b.fileName && textSpansEqual(a.textSpan, b.textSpan);
    }

    /**
     * Iterates through 'array' by index and performs the callback on each element of array until the callback
     * returns a truthy value, then returns that value.
     * If no such value is found, the callback is applied to each element of array and undefined is returned.
     */
    export function forEachUnique<T, U>(array: readonly T[] | undefined, callback: (element: T, index: number) => U): U | undefined {
        if (array) {
            for (let i = 0; i < array.length; i++) {
                if (array.indexOf(array[i]) === i) {
                    const result = callback(array[i], i);
                    if (result) {
                        return result;
                    }
                }
            }
        }
        return undefined;
    }

    // #endregion

    // Display-part writer helpers
    // #region
    export function isFirstDeclarationOfSymbolParameter(symbol: Symbol) {
        return symbol.declarations && symbol.declarations.length > 0 && symbol.declarations[0].kind === SyntaxKind.Parameter;
    }

    const displayPartWriter = getDisplayPartWriter();
    function getDisplayPartWriter(): DisplayPartsSymbolWriter {
        const absoluteMaximumLength = defaultMaximumTruncationLength * 10; // A hard cutoff to avoid overloading the messaging channel in worst-case scenarios
        let displayParts: SymbolDisplayPart[];
        let lineStart: boolean;
        let indent: number;
        let length: number;

        resetWriter();
        const unknownWrite = (text: string) => writeKind(text, SymbolDisplayPartKind.text);
        return {
            displayParts: () => {
                const finalText = displayParts.length && displayParts[displayParts.length - 1].text;
                if (length > absoluteMaximumLength && finalText && finalText !== "...") {
                    if (!isWhiteSpaceLike(finalText.charCodeAt(finalText.length - 1))) {
                        displayParts.push(displayPart(" ", SymbolDisplayPartKind.space));
                    }
                    displayParts.push(displayPart("...", SymbolDisplayPartKind.punctuation));
                }
                return displayParts;
            },
            writeKeyword: text => writeKind(text, SymbolDisplayPartKind.keyword),
            writeOperator: text => writeKind(text, SymbolDisplayPartKind.operator),
            writePunctuation: text => writeKind(text, SymbolDisplayPartKind.punctuation),
            writeTrailingSemicolon: text => writeKind(text, SymbolDisplayPartKind.punctuation),
            writeSpace: text => writeKind(text, SymbolDisplayPartKind.space),
            writeStringLiteral: text => writeKind(text, SymbolDisplayPartKind.stringLiteral),
            writeParameter: text => writeKind(text, SymbolDisplayPartKind.parameterName),
            writeProperty: text => writeKind(text, SymbolDisplayPartKind.propertyName),
            writeLiteral: text => writeKind(text, SymbolDisplayPartKind.stringLiteral),
            writeSymbol,
            writeLine,
            write: unknownWrite,
            writeComment: unknownWrite,
            getText: () => "",
            getTextPos: () => 0,
            getColumn: () => 0,
            getLine: () => 0,
            isAtStartOfLine: () => false,
            hasTrailingWhitespace: () => false,
            hasTrailingComment: () => false,
            rawWrite: notImplemented,
            getIndent: () => indent,
            increaseIndent: () => { indent++; },
            decreaseIndent: () => { indent--; },
            clear: resetWriter,
            trackSymbol: noop,
            reportInaccessibleThisError: noop,
            reportInaccessibleUniqueSymbolError: noop,
            reportPrivateInBaseOfClassExpression: noop,
        };

        function writeIndent() {
            if (length > absoluteMaximumLength) return;
            if (lineStart) {
                const indentString = getIndentString(indent);
                if (indentString) {
                    length += indentString.length;
                    displayParts.push(displayPart(indentString, SymbolDisplayPartKind.space));
                }
                lineStart = false;
            }
        }

        function writeKind(text: string, kind: SymbolDisplayPartKind) {
            if (length > absoluteMaximumLength) return;
            writeIndent();
            length += text.length;
            displayParts.push(displayPart(text, kind));
        }

        function writeSymbol(text: string, symbol: Symbol) {
            if (length > absoluteMaximumLength) return;
            writeIndent();
            length += text.length;
            displayParts.push(symbolPart(text, symbol));
        }

        function writeLine() {
            if (length > absoluteMaximumLength) return;
            length += 1;
            displayParts.push(lineBreakPart());
            lineStart = true;
        }

        function resetWriter() {
            displayParts = [];
            lineStart = true;
            indent = 0;
            length = 0;
        }
    }

    export function symbolPart(text: string, symbol: Symbol) {
        return displayPart(text, displayPartKind(symbol));

        function displayPartKind(symbol: Symbol): SymbolDisplayPartKind {
            const flags = symbol.flags;

            if (flags & SymbolFlags.Variable) {
                return isFirstDeclarationOfSymbolParameter(symbol) ? SymbolDisplayPartKind.parameterName : SymbolDisplayPartKind.localName;
            }
            else if (flags & SymbolFlags.Property) { return SymbolDisplayPartKind.propertyName; }
            else if (flags & SymbolFlags.GetAccessor) { return SymbolDisplayPartKind.propertyName; }
            else if (flags & SymbolFlags.SetAccessor) { return SymbolDisplayPartKind.propertyName; }
            else if (flags & SymbolFlags.EnumMember) { return SymbolDisplayPartKind.enumMemberName; }
            else if (flags & SymbolFlags.Function) { return SymbolDisplayPartKind.functionName; }
            else if (flags & SymbolFlags.Class) { return SymbolDisplayPartKind.className; }
            else if (flags & SymbolFlags.Interface) { return SymbolDisplayPartKind.interfaceName; }
            else if (flags & SymbolFlags.Enum) { return SymbolDisplayPartKind.enumName; }
            else if (flags & SymbolFlags.Module) { return SymbolDisplayPartKind.moduleName; }
            else if (flags & SymbolFlags.Method) { return SymbolDisplayPartKind.methodName; }
            else if (flags & SymbolFlags.TypeParameter) { return SymbolDisplayPartKind.typeParameterName; }
            else if (flags & SymbolFlags.TypeAlias) { return SymbolDisplayPartKind.aliasName; }
            else if (flags & SymbolFlags.Alias) { return SymbolDisplayPartKind.aliasName; }

            return SymbolDisplayPartKind.text;
        }
    }

    export function displayPart(text: string, kind: SymbolDisplayPartKind): SymbolDisplayPart {
        return { text, kind: SymbolDisplayPartKind[kind] };
    }

    export function spacePart() {
        return displayPart(" ", SymbolDisplayPartKind.space);
    }

    export function keywordPart(kind: SyntaxKind) {
        return displayPart(tokenToString(kind)!, SymbolDisplayPartKind.keyword);
    }

    export function punctuationPart(kind: SyntaxKind) {
        return displayPart(tokenToString(kind)!, SymbolDisplayPartKind.punctuation);
    }

    export function operatorPart(kind: SyntaxKind) {
        return displayPart(tokenToString(kind)!, SymbolDisplayPartKind.operator);
    }

    export function textOrKeywordPart(text: string) {
        const kind = stringToToken(text);
        return kind === undefined
            ? textPart(text)
            : keywordPart(kind);
    }

    export function textPart(text: string) {
        return displayPart(text, SymbolDisplayPartKind.text);
    }

    const carriageReturnLineFeed = "\r\n";
    /**
     * The default is CRLF.
     */
    export function getNewLineOrDefaultFromHost(host: LanguageServiceHost | LanguageServiceShimHost, formatSettings?: FormatCodeSettings) {
        return (formatSettings && formatSettings.newLineCharacter) ||
            (host.getNewLine && host.getNewLine()) ||
            carriageReturnLineFeed;
    }

    export function lineBreakPart() {
        return displayPart("\n", SymbolDisplayPartKind.lineBreak);
    }

    export function mapToDisplayParts(writeDisplayParts: (writer: DisplayPartsSymbolWriter) => void): SymbolDisplayPart[] {
        try {
            writeDisplayParts(displayPartWriter);
            return displayPartWriter.displayParts();
        }
        finally {
            displayPartWriter.clear();
        }
    }

    export function typeToDisplayParts(typechecker: TypeChecker, type: Type, enclosingDeclaration?: Node, flags: TypeFormatFlags = TypeFormatFlags.None): SymbolDisplayPart[] {
        return mapToDisplayParts(writer => {
            typechecker.writeType(type, enclosingDeclaration, flags | TypeFormatFlags.MultilineObjectLiterals | TypeFormatFlags.UseAliasDefinedOutsideCurrentScope, writer);
        });
    }

    export function symbolToDisplayParts(typeChecker: TypeChecker, symbol: Symbol, enclosingDeclaration?: Node, meaning?: SymbolFlags, flags: SymbolFormatFlags = SymbolFormatFlags.None): SymbolDisplayPart[] {
        return mapToDisplayParts(writer => {
            typeChecker.writeSymbol(symbol, enclosingDeclaration, meaning, flags | SymbolFormatFlags.UseAliasDefinedOutsideCurrentScope, writer);
        });
    }

    export function signatureToDisplayParts(typechecker: TypeChecker, signature: Signature, enclosingDeclaration?: Node, flags: TypeFormatFlags = TypeFormatFlags.None): SymbolDisplayPart[] {
        flags |= TypeFormatFlags.UseAliasDefinedOutsideCurrentScope | TypeFormatFlags.MultilineObjectLiterals | TypeFormatFlags.WriteTypeArgumentsOfSignature | TypeFormatFlags.OmitParameterModifiers;
        return mapToDisplayParts(writer => {
            typechecker.writeSignature(signature, enclosingDeclaration, flags, /*signatureKind*/ undefined, writer);
        });
    }

    export function isImportOrExportSpecifierName(location: Node): location is Identifier {
        return !!location.parent && isImportOrExportSpecifier(location.parent) && location.parent.propertyName === location;
    }

    export function scriptKindIs(fileName: string, host: LanguageServiceHost, ...scriptKinds: ScriptKind[]): boolean {
        const scriptKind = getScriptKind(fileName, host);
        return some(scriptKinds, k => k === scriptKind);
    }

    export function getScriptKind(fileName: string, host?: LanguageServiceHost): ScriptKind {
        // First check to see if the script kind was specified by the host. Chances are the host
        // may override the default script kind for the file extension.
        return ensureScriptKind(fileName, host && host.getScriptKind && host.getScriptKind(fileName));
    }

    export function getSymbolTarget(symbol: Symbol, checker: TypeChecker): Symbol {
        let next: Symbol = symbol;
        while (isAliasSymbol(next) || (isTransientSymbol(next) && next.target)) {
            if (isTransientSymbol(next) && next.target) {
                next = next.target;
            }
            else {
                next = skipAlias(next, checker);
            }
        }
        return next;
    }

    function isTransientSymbol(symbol: Symbol): symbol is TransientSymbol {
        return (symbol.flags & SymbolFlags.Transient) !== 0;
    }

    function isAliasSymbol(symbol: Symbol): boolean {
        return (symbol.flags & SymbolFlags.Alias) !== 0;
    }

    export function getUniqueSymbolId(symbol: Symbol, checker: TypeChecker) {
        return getSymbolId(skipAlias(symbol, checker));
    }

    export function getFirstNonSpaceCharacterPosition(text: string, position: number) {
        while (isWhiteSpaceLike(text.charCodeAt(position))) {
            position += 1;
        }
        return position;
    }

    export function getPrecedingNonSpaceCharacterPosition(text: string, position: number) {
        while (position > -1 && isWhiteSpaceSingleLine(text.charCodeAt(position))) {
            position -= 1;
        }
        return position + 1;
    }

    /**
     * Creates a deep, memberwise clone of a node with no source map location.
     *
     * WARNING: This is an expensive operation and is only intended to be used in refactorings
     * and code fixes (because those are triggered by explicit user actions).
     */
    export function getSynthesizedDeepClone<T extends Node | undefined>(node: T, includeTrivia = true): T {
        const clone = node && getSynthesizedDeepCloneWorker(node as NonNullable<T>);
        if (clone && !includeTrivia) suppressLeadingAndTrailingTrivia(clone);
        return clone;
    }

    export function getSynthesizedDeepCloneWithRenames<T extends Node>(node: T, includeTrivia = true, renameMap?: Map<Identifier>, checker?: TypeChecker, callback?: (originalNode: Node, clone: Node) => any): T {
        let clone;
        if (renameMap && checker && isBindingElement(node) && isIdentifier(node.name) && isObjectBindingPattern(node.parent)) {
            const symbol = checker.getSymbolAtLocation(node.name);
            const renameInfo = symbol && renameMap.get(String(getSymbolId(symbol)));

            if (renameInfo && renameInfo.text !== (node.name || node.propertyName).getText()) {
                clone = createBindingElement(
                    node.dotDotDotToken,
                    node.propertyName || node.name,
                    renameInfo,
                    node.initializer);
            }
        }
        else if (renameMap && checker && isIdentifier(node)) {
            const symbol = checker.getSymbolAtLocation(node);
            const renameInfo = symbol && renameMap.get(String(getSymbolId(symbol)));

            if (renameInfo) {
                clone = createIdentifier(renameInfo.text);
            }
        }

        if (!clone) {
            clone = getSynthesizedDeepCloneWorker(node as NonNullable<T>, renameMap, checker, callback);
        }

        if (clone && !includeTrivia) suppressLeadingAndTrailingTrivia(clone);
        if (callback && clone) callback(node, clone);

        return clone as T;
    }


    function getSynthesizedDeepCloneWorker<T extends Node>(node: T, renameMap?: Map<Identifier>, checker?: TypeChecker, callback?: (originalNode: Node, clone: Node) => any): T {
        const visited = (renameMap || checker || callback) ?
            visitEachChild(node, wrapper, nullTransformationContext) :
            visitEachChild(node, getSynthesizedDeepClone, nullTransformationContext);

        if (visited === node) {
            // This only happens for leaf nodes - internal nodes always see their children change.
            const clone = getSynthesizedClone(node);
            if (isStringLiteral(clone)) {
                clone.textSourceNode = node as any;
            }
            else if (isNumericLiteral(clone)) {
                clone.numericLiteralFlags = (node as any).numericLiteralFlags;
            }
            return setTextRange(clone, node);
        }

        // PERF: As an optimization, rather than calling getSynthesizedClone, we'll update
        // the new node created by visitEachChild with the extra changes getSynthesizedClone
        // would have made.
        visited.parent = undefined!;
        return visited;

        function wrapper(node: T) {
            return getSynthesizedDeepCloneWithRenames(node, /*includeTrivia*/ true, renameMap, checker, callback);
        }
    }

    export function getSynthesizedDeepClones<T extends Node>(nodes: NodeArray<T>, includeTrivia?: boolean): NodeArray<T>;
    export function getSynthesizedDeepClones<T extends Node>(nodes: NodeArray<T> | undefined, includeTrivia?: boolean): NodeArray<T> | undefined;
    export function getSynthesizedDeepClones<T extends Node>(nodes: NodeArray<T> | undefined, includeTrivia = true): NodeArray<T> | undefined {
        return nodes && createNodeArray(nodes.map(n => getSynthesizedDeepClone(n, includeTrivia)), nodes.hasTrailingComma);
    }

    /**
     * Sets EmitFlags to suppress leading and trailing trivia on the node.
     */
    export function suppressLeadingAndTrailingTrivia(node: Node) {
        suppressLeadingTrivia(node);
        suppressTrailingTrivia(node);
    }

    /**
     * Sets EmitFlags to suppress leading trivia on the node.
     */
    export function suppressLeadingTrivia(node: Node) {
        addEmitFlagsRecursively(node, EmitFlags.NoLeadingComments, getFirstChild);
    }

    /**
     * Sets EmitFlags to suppress trailing trivia on the node.
     */
    export function suppressTrailingTrivia(node: Node) {
        addEmitFlagsRecursively(node, EmitFlags.NoTrailingComments, getLastChild);
    }

    function addEmitFlagsRecursively(node: Node, flag: EmitFlags, getChild: (n: Node) => Node | undefined) {
        addEmitFlags(node, flag);
        const child = getChild(node);
        if (child) addEmitFlagsRecursively(child, flag, getChild);
    }

    function getFirstChild(node: Node): Node | undefined {
        return node.forEachChild(child => child);
    }

    export function getUniqueName(baseName: string, sourceFile: SourceFile): string {
        let nameText = baseName;
        for (let i = 1; !isFileLevelUniqueName(sourceFile, nameText); i++) {
            nameText = `${baseName}_${i}`;
        }
        return nameText;
    }

    /**
     * @return The index of the (only) reference to the extracted symbol.  We want the cursor
     * to be on the reference, rather than the declaration, because it's closer to where the
     * user was before extracting it.
     */
    export function getRenameLocation(edits: readonly FileTextChanges[], renameFilename: string, name: string, preferLastLocation: boolean): number {
        let delta = 0;
        let lastPos = -1;
        for (const { fileName, textChanges } of edits) {
            Debug.assert(fileName === renameFilename);
            for (const change of textChanges) {
                const { span, newText } = change;
                const index = indexInTextChange(newText, name);
                if (index !== -1) {
                    lastPos = span.start + delta + index;

                    // If the reference comes first, return immediately.
                    if (!preferLastLocation) {
                        return lastPos;
                    }
                }
                delta += newText.length - span.length;
            }
        }

        // If the declaration comes first, return the position of the last occurrence.
        Debug.assert(preferLastLocation);
        Debug.assert(lastPos >= 0);
        return lastPos;
    }

    export function copyLeadingComments(sourceNode: Node, targetNode: Node, sourceFile: SourceFile, commentKind?: CommentKind, hasTrailingNewLine?: boolean) {
        forEachLeadingCommentRange(sourceFile.text, sourceNode.pos, getAddCommentsFunction(targetNode, sourceFile, commentKind, hasTrailingNewLine, addSyntheticLeadingComment));
    }


    export function copyTrailingComments(sourceNode: Node, targetNode: Node, sourceFile: SourceFile, commentKind?: CommentKind, hasTrailingNewLine?: boolean) {
        forEachTrailingCommentRange(sourceFile.text, sourceNode.end, getAddCommentsFunction(targetNode, sourceFile, commentKind, hasTrailingNewLine, addSyntheticTrailingComment));
    }

    /**
     * This function copies the trailing comments for the token that comes before `sourceNode`, as leading comments of `targetNode`.
     * This is useful because sometimes a comment that refers to `sourceNode` will be a leading comment for `sourceNode`, according to the
     * notion of trivia ownership, and instead will be a trailing comment for the token before `sourceNode`, e.g.:
     * `function foo(\* not leading comment for a *\ a: string) {}`
     * The comment refers to `a` but belongs to the `(` token, but we might want to copy it.
     */
    export function copyTrailingAsLeadingComments(sourceNode: Node, targetNode: Node, sourceFile: SourceFile, commentKind?: CommentKind, hasTrailingNewLine?: boolean) {
        forEachTrailingCommentRange(sourceFile.text, sourceNode.pos, getAddCommentsFunction(targetNode, sourceFile, commentKind, hasTrailingNewLine, addSyntheticLeadingComment));
    }

    function getAddCommentsFunction(targetNode: Node, sourceFile: SourceFile, commentKind: CommentKind | undefined, hasTrailingNewLine: boolean | undefined, cb: (node: Node, kind: CommentKind, text: string, hasTrailingNewLine?: boolean) => void) {
        return (pos: number, end: number, kind: CommentKind, htnl: boolean) => {
            if (kind === SyntaxKind.MultiLineCommentTrivia) {
                // Remove leading /*
                pos += 2;
                // Remove trailing */
                end -= 2;
            }
            else {
                // Remove leading //
                pos += 2;
            }
            cb(targetNode, commentKind || kind, sourceFile.text.slice(pos, end), hasTrailingNewLine !== undefined ? hasTrailingNewLine : htnl);
        };
    }

    function indexInTextChange(change: string, name: string): number {
        if (startsWith(change, name)) return 0;
        // Add a " " to avoid references inside words
        let idx = change.indexOf(" " + name);
        if (idx === -1) idx = change.indexOf("." + name);
        if (idx === -1) idx = change.indexOf('"' + name);
        return idx === -1 ? -1 : idx + 1;
    }

    export function getContextualTypeFromParent(node: Expression, checker: TypeChecker): Type | undefined {
        const { parent } = node;
        switch (parent.kind) {
            case SyntaxKind.NewExpression:
                return checker.getContextualType(parent as NewExpression);
            case SyntaxKind.BinaryExpression: {
                const { left, operatorToken, right } = parent as BinaryExpression;
                return isEqualityOperatorKind(operatorToken.kind)
                    ? checker.getTypeAtLocation(node === right ? left : right)
                    : checker.getContextualType(node);
            }
            case SyntaxKind.CaseClause:
                return (parent as CaseClause).expression === node ? getSwitchedType(parent as CaseClause, checker) : undefined;
            default:
                return checker.getContextualType(node);
        }
    }

    export function quote(text: string, preferences: UserPreferences): string {
        if (/^\d+$/.test(text)) {
            return text;
        }
        // Editors can pass in undefined or empty string - we want to infer the preference in those cases.
        const quotePreference = preferences.quotePreference || "auto";
        const quoted = JSON.stringify(text);
        switch (quotePreference) {
            // TODO use getQuotePreference to infer the actual quote style.
            case "auto":
            case "double":
                return quoted;
            case "single":
                return `'${stripQuotes(quoted).replace("'", "\\'").replace('\\"', '"')}'`;
            default:
                return Debug.assertNever(quotePreference);
        }
    }

    export function isEqualityOperatorKind(kind: SyntaxKind): kind is EqualityOperator {
        switch (kind) {
            case SyntaxKind.EqualsEqualsEqualsToken:
            case SyntaxKind.EqualsEqualsToken:
            case SyntaxKind.ExclamationEqualsEqualsToken:
            case SyntaxKind.ExclamationEqualsToken:
                return true;
            default:
                return false;
        }
    }

    export function isStringLiteralOrTemplate(node: Node): node is StringLiteralLike | TemplateExpression | TaggedTemplateExpression {
        switch (node.kind) {
            case SyntaxKind.StringLiteral:
            case SyntaxKind.NoSubstitutionTemplateLiteral:
            case SyntaxKind.TemplateExpression:
            case SyntaxKind.TaggedTemplateExpression:
                return true;
            default:
                return false;
        }
    }

    export function hasIndexSignature(type: Type): boolean {
        return !!type.getStringIndexType() || !!type.getNumberIndexType();
    }

    export function getSwitchedType(caseClause: CaseClause, checker: TypeChecker): Type | undefined {
        return checker.getTypeAtLocation(caseClause.parent.parent.expression);
    }

    export function getTypeNodeIfAccessible(type: Type, enclosingScope: Node, program: Program, host: LanguageServiceHost): TypeNode | undefined {
        const checker = program.getTypeChecker();
        let typeIsAccessible = true;
        const notAccessible = () => { typeIsAccessible = false; };
        const res = checker.typeToTypeNode(type, enclosingScope, /*flags*/ undefined, {
            trackSymbol: (symbol, declaration, meaning) => {
                typeIsAccessible = typeIsAccessible && checker.isSymbolAccessible(symbol, declaration, meaning, /*shouldComputeAliasToMarkVisible*/ false).accessibility === SymbolAccessibility.Accessible;
            },
            reportInaccessibleThisError: notAccessible,
            reportPrivateInBaseOfClassExpression: notAccessible,
            reportInaccessibleUniqueSymbolError: notAccessible,
            moduleResolverHost: {
                readFile: host.readFile,
                fileExists: host.fileExists,
                directoryExists: host.directoryExists,
                getSourceFiles: program.getSourceFiles,
                getCurrentDirectory: program.getCurrentDirectory,
                getCommonSourceDirectory: program.getCommonSourceDirectory,
            }
        });
        return typeIsAccessible ? res : undefined;
    }

    export function syntaxRequiresTrailingCommaOrSemicolonOrASI(kind: SyntaxKind) {
        return kind === SyntaxKind.CallSignature
            || kind === SyntaxKind.ConstructSignature
            || kind === SyntaxKind.IndexSignature
            || kind === SyntaxKind.PropertySignature
            || kind === SyntaxKind.MethodSignature;
    }

    export function syntaxRequiresTrailingFunctionBlockOrSemicolonOrASI(kind: SyntaxKind) {
        return kind === SyntaxKind.FunctionDeclaration
            || kind === SyntaxKind.Constructor
            || kind === SyntaxKind.MethodDeclaration
            || kind === SyntaxKind.GetAccessor
            || kind === SyntaxKind.SetAccessor;
    }

    export function syntaxRequiresTrailingModuleBlockOrSemicolonOrASI(kind: SyntaxKind) {
        return kind === SyntaxKind.ModuleDeclaration;
    }

    export function syntaxRequiresTrailingSemicolonOrASI(kind: SyntaxKind) {
        return kind === SyntaxKind.VariableStatement
            || kind === SyntaxKind.ExpressionStatement
            || kind === SyntaxKind.DoStatement
            || kind === SyntaxKind.ContinueStatement
            || kind === SyntaxKind.BreakStatement
            || kind === SyntaxKind.ReturnStatement
            || kind === SyntaxKind.ThrowStatement
            || kind === SyntaxKind.DebuggerStatement
            || kind === SyntaxKind.PropertyDeclaration
            || kind === SyntaxKind.TypeAliasDeclaration
            || kind === SyntaxKind.ImportDeclaration
            || kind === SyntaxKind.ImportEqualsDeclaration
            || kind === SyntaxKind.ExportDeclaration
            || kind === SyntaxKind.NamespaceExportDeclaration
            || kind === SyntaxKind.ExportAssignment;
    }

    export const syntaxMayBeASICandidate = or(
        syntaxRequiresTrailingCommaOrSemicolonOrASI,
        syntaxRequiresTrailingFunctionBlockOrSemicolonOrASI,
        syntaxRequiresTrailingModuleBlockOrSemicolonOrASI,
        syntaxRequiresTrailingSemicolonOrASI);

    function nodeIsASICandidate(node: Node, sourceFile: SourceFileLike): boolean {
        const lastToken = node.getLastToken(sourceFile);
        if (lastToken && lastToken.kind === SyntaxKind.SemicolonToken) {
            return false;
        }

        if (syntaxRequiresTrailingCommaOrSemicolonOrASI(node.kind)) {
            if (lastToken && lastToken.kind === SyntaxKind.CommaToken) {
                return false;
            }
        }
        else if (syntaxRequiresTrailingModuleBlockOrSemicolonOrASI(node.kind)) {
            const lastChild = last(node.getChildren(sourceFile));
            if (lastChild && isModuleBlock(lastChild)) {
                return false;
            }
        }
        else if (syntaxRequiresTrailingFunctionBlockOrSemicolonOrASI(node.kind)) {
            const lastChild = last(node.getChildren(sourceFile));
            if (lastChild && isFunctionBlock(lastChild)) {
                return false;
            }
        }
        else if (!syntaxRequiresTrailingSemicolonOrASI(node.kind)) {
            return false;
        }

        // See comment in parser’s `parseDoStatement`
        if (node.kind === SyntaxKind.DoStatement) {
            return true;
        }

        const topNode = findAncestor(node, ancestor => !ancestor.parent)!;
        const nextToken = findNextToken(node, topNode, sourceFile);
        if (!nextToken || nextToken.kind === SyntaxKind.CloseBraceToken) {
            return true;
        }

        const startLine = sourceFile.getLineAndCharacterOfPosition(node.getEnd()).line;
        const endLine = sourceFile.getLineAndCharacterOfPosition(nextToken.getStart(sourceFile)).line;
        return startLine !== endLine;
    }

    export function positionIsASICandidate(pos: number, context: Node, sourceFile: SourceFileLike): boolean {
        const contextAncestor = findAncestor(context, ancestor => {
            if (ancestor.end !== pos) {
                return "quit";
            }
            return syntaxMayBeASICandidate(ancestor.kind);
        });

        return !!contextAncestor && nodeIsASICandidate(contextAncestor, sourceFile);
    }

    export function probablyUsesSemicolons(sourceFile: SourceFile): boolean {
        let withSemicolon = 0;
        let withoutSemicolon = 0;
        const nStatementsToObserve = 5;
        forEachChild(sourceFile, function visit(node): boolean | undefined {
            if (syntaxRequiresTrailingSemicolonOrASI(node.kind)) {
                const lastToken = node.getLastToken(sourceFile);
                if (lastToken && lastToken.kind === SyntaxKind.SemicolonToken) {
                    withSemicolon++;
                }
                else {
                    withoutSemicolon++;
                }
            }
            if (withSemicolon + withoutSemicolon >= nStatementsToObserve) {
                return true;
            }

            return forEachChild(node, visit);
        });

        // One statement missing a semicolon isn’t sufficient evidence to say the user
        // doesn’t want semicolons, because they may not even be done writing that statement.
        if (withSemicolon === 0 && withoutSemicolon <= 1) {
            return true;
        }

        // If even 2/5 places have a semicolon, the user probably wants semicolons
        return withSemicolon / withoutSemicolon > 1 / nStatementsToObserve;
    }

    export function tryGetDirectories(host: Pick<LanguageServiceHost, "getDirectories">, directoryName: string): string[] {
        return tryIOAndConsumeErrors(host, host.getDirectories, directoryName) || [];
    }

    export function tryReadDirectory(host: Pick<LanguageServiceHost, "readDirectory">, path: string, extensions?: readonly string[], exclude?: readonly string[], include?: readonly string[]): readonly string[] {
        return tryIOAndConsumeErrors(host, host.readDirectory, path, extensions, exclude, include) || emptyArray;
    }

    export function tryFileExists(host: Pick<LanguageServiceHost, "fileExists">, path: string): boolean {
        return tryIOAndConsumeErrors(host, host.fileExists, path);
    }

    export function tryDirectoryExists(host: LanguageServiceHost, path: string): boolean {
        return tryAndIgnoreErrors(() => directoryProbablyExists(path, host)) || false;
    }

    export function tryAndIgnoreErrors<T>(cb: () => T): T | undefined {
        try { return cb(); }
        catch { return undefined; }
    }

    export function tryIOAndConsumeErrors<T>(host: unknown, toApply: ((...a: any[]) => T) | undefined, ...args: any[]) {
        return tryAndIgnoreErrors(() => toApply && toApply.apply(host, args));
    }

    export function findPackageJsons(startDirectory: string, host: Pick<LanguageServiceHost, "fileExists">, stopDirectory?: string): string[] {
        const paths: string[] = [];
        forEachAncestorDirectory(startDirectory, ancestor => {
            if (ancestor === stopDirectory) {
                return true;
            }
            const currentConfigPath = combinePaths(ancestor, "package.json");
            if (tryFileExists(host, currentConfigPath)) {
                paths.push(currentConfigPath);
            }
        });
        return paths;
    }

    export function findPackageJson(directory: string, host: LanguageServiceHost): string | undefined {
        let packageJson: string | undefined;
        forEachAncestorDirectory(directory, ancestor => {
            if (ancestor === "node_modules") return true;
            packageJson = findConfigFile(ancestor, (f) => tryFileExists(host, f), "package.json");
            if (packageJson) {
                return true; // break out
            }
        });
        return packageJson;
    }

    export function getPackageJsonsVisibleToFile(fileName: string, host: LanguageServiceHost): readonly PackageJsonInfo[] {
        if (!host.fileExists) {
            return [];
        }

        const packageJsons: PackageJsonInfo[] = [];
        forEachAncestorDirectory(getDirectoryPath(fileName), ancestor => {
            const packageJsonFileName = combinePaths(ancestor, "package.json");
            if (host.fileExists!(packageJsonFileName)) {
                const info = createPackageJsonInfo(packageJsonFileName, host);
                if (info) {
                    packageJsons.push(info);
                }
            }
        });

        return packageJsons;
    }

    export function createPackageJsonInfo(fileName: string, host: LanguageServiceHost): PackageJsonInfo | false | undefined {
        if (!host.readFile) {
            return undefined;
        }

        type PackageJsonRaw = Record<typeof dependencyKeys[number], Record<string, string> | undefined>;
        const dependencyKeys = ["dependencies", "devDependencies", "optionalDependencies", "peerDependencies"] as const;
        const stringContent = host.readFile(fileName);
        if (!stringContent) return undefined;

        const content = tryParseJson(stringContent) as PackageJsonRaw;
        if (!content) return false;
        const info: Pick<PackageJsonInfo, typeof dependencyKeys[number]> = {};
        for (const key of dependencyKeys) {
            const dependencies = content[key];
            if (!dependencies) {
                continue;
            }
            const dependencyMap = createMap<string>();
            for (const packageName in dependencies) {
                dependencyMap.set(packageName, dependencies[packageName]);
            }
            info[key] = dependencyMap;
        }

        const dependencyGroups = [
            [PackageJsonDependencyGroup.Dependencies, info.dependencies],
            [PackageJsonDependencyGroup.DevDependencies, info.devDependencies],
            [PackageJsonDependencyGroup.OptionalDependencies, info.optionalDependencies],
            [PackageJsonDependencyGroup.PeerDependencies, info.peerDependencies],
        ] as const;

        return {
            ...info,
            fileName,
            get,
            has(dependencyName, inGroups) {
                return !!get(dependencyName, inGroups);
            },
        };

        function get(dependencyName: string, inGroups = PackageJsonDependencyGroup.All) {
            for (const [group, deps] of dependencyGroups) {
                if (deps && (inGroups & group)) {
                    const dep = deps.get(dependencyName);
                    if (dep !== undefined) {
                        return dep;
                    }
                }
            }
        }
    }

    function tryParseJson(text: string) {
        try {
            return JSON.parse(text);
        }
        catch {
            return undefined;
        }
    }

    export function consumesNodeCoreModules(sourceFile: SourceFile): boolean {
        return some(sourceFile.imports, ({ text }) => JsTyping.nodeCoreModules.has(text));
    }

    export function isInsideNodeModules(fileOrDirectory: string): boolean {
        return contains(getPathComponents(fileOrDirectory), "node_modules");
    }

    export function isDiagnosticWithLocation(diagnostic: Diagnostic): diagnostic is DiagnosticWithLocation {
        return diagnostic.file !== undefined && diagnostic.start !== undefined && diagnostic.length !== undefined;
    }

    export function findDiagnosticForNode(node: Node, sortedFileDiagnostics: readonly Diagnostic[]): DiagnosticWithLocation | undefined {
        const span: Partial<TextSpan> = createTextSpanFromNode(node);
        const index = binarySearchKey(sortedFileDiagnostics, span, identity, compareTextSpans);
        if (index >= 0) {
            const diagnostic = sortedFileDiagnostics[index];
            Debug.assertEqual(diagnostic.file, node.getSourceFile(), "Diagnostics proided to 'findDiagnosticForNode' must be from a single SourceFile");
            return cast(diagnostic, isDiagnosticWithLocation);
        }
    }

    export function getDiagnosticsWithinSpan(span: TextSpan, sortedFileDiagnostics: readonly Diagnostic[]): readonly DiagnosticWithLocation[] {
        let index = binarySearchKey(sortedFileDiagnostics, span.start, diag => diag.start, compareValues);
        if (index < 0) {
            index = ~index;
        }
        while (sortedFileDiagnostics[index - 1]?.start === span.start) {
            index--;
        }

        const result: DiagnosticWithLocation[] = [];
        const end = textSpanEnd(span);
        while (true) {
            const diagnostic = tryCast(sortedFileDiagnostics[index], isDiagnosticWithLocation);
            if (!diagnostic || diagnostic.start > end) {
                break;
            }
            if (textSpanContainsTextSpan(span, diagnostic)) {
                result.push(diagnostic);
            }
            index++;
        }

        return result;
    }

    /* @internal */
    export function getRefactorContextSpan({ startPosition, endPosition }: RefactorContext): TextSpan {
        return createTextSpanFromBounds(startPosition, endPosition === undefined ? startPosition : endPosition);
    }

    /**
     * If the provided value is an array, the mapping function is applied to each element; otherwise, the mapping function is applied
     * to the provided value itself.
     */
    export function mapOneOrMany<T, U>(valueOrArray: T | readonly T[], f: (x: T, i: number) => U): U | U[];
    export function mapOneOrMany<T, U>(valueOrArray: T | readonly T[] | undefined, f: (x: T, i: number) => U): U | U[] | undefined;
    export function mapOneOrMany<T, U>(valueOrArray: T | readonly T[], f: (x: T, i: number) => U, resultSelector: (x: U[]) => U): U;
    export function mapOneOrMany<T, U>(valueOrArray: T | readonly T[] | undefined, f: (x: T, i: number) => U, resultSelector: (x: U[]) => U): U | undefined;
    export function mapOneOrMany<T, U>(valueOrArray: T | readonly T[] | undefined, f: (x: T, i: number) => U, resultSelector: (x: U[]) => U | U[] = identity): U | U[] | undefined {
        return valueOrArray ? isArray(valueOrArray) ? resultSelector(map(valueOrArray, f)) : f(valueOrArray, 0) : undefined;
    }

    /**
     * If the provided value is an array, the first element of the array is returned; otherwise, the provided value is returned instead.
     */
    export function firstOrOnly<T>(valueOrArray: T | readonly T[]): T {
        return isArray(valueOrArray) ? first(valueOrArray) : valueOrArray;
    }

    // #endregion
}
