Insert async keyword as last modifier (#27491)

src/services/codefixes/convertToAsyncFunction.ts

@@ -68,7 +68,7 @@ namespace ts.codefix {
         }
 
         // add the async keyword
-        changes.insertModifierBefore(sourceFile, SyntaxKind.AsyncKeyword, functionToConvert);
+        changes.insertLastModifierBefore(sourceFile, SyntaxKind.AsyncKeyword, functionToConvert);
 
         function startTransformation(node: CallExpression, nodeToReplace: Node) {
             const newNodes = transformExpression(node, transformer, node);

src/services/textChanges.ts

@@ -315,6 +315,16 @@ namespace ts.textChanges {
             this.replaceRange(sourceFile, { pos, end: pos }, createToken(modifier), { suffix: " " });
         }
 
+        public insertLastModifierBefore(sourceFile: SourceFile, modifier: SyntaxKind, before: Node): void {
+            if (!before.modifiers) {
+                this.insertModifierBefore(sourceFile, modifier, before);
+                return;
+            }
+
+            const pos = before.modifiers.end;
+            this.replaceRange(sourceFile, { pos, end: pos }, createToken(modifier), { prefix: " " });
+        }
+
         public insertCommentBeforeLine(sourceFile: SourceFile, lineNumber: number, position: number, commentText: string): void {
             const lineStartPosition = getStartPositionOfLine(lineNumber, sourceFile);
             const startPosition = getFirstNonSpaceCharacterPosition(sourceFile.text, lineStartPosition);

src/testRunner/unittests/convertToAsyncFunction.ts

@@ -1254,6 +1254,11 @@ function [#|main2|]() {
         .then(() => { console.log("."); return delay(500); })
         .then(() => { console.log("."); return delay(500); })
 }
+`);
+_testConvertToAsyncFunction("convertToAsyncFunction_exportModifier", `
+export function [#|foo|]() {
+    return fetch('https://typescriptlang.org').then(s => console.log(s));
+}
 `);
     });
 

tests/baselines/reference/convertToAsyncFunction/convertToAsyncFunction_exportModifier.js

@@ -0,0 +1,12 @@
+// ==ORIGINAL==
+
+export function /*[#|*/foo/*|]*/() {
+    return fetch('https://typescriptlang.org').then(s => console.log(s));
+}
+
+// ==ASYNC FUNCTION::Convert to async function==
+
+export async function foo() {
+    const s = await fetch('https://typescriptlang.org');
+    return console.log(s);
+}

tests/baselines/reference/convertToAsyncFunction/convertToAsyncFunction_exportModifier.ts

@@ -0,0 +1,12 @@
+// ==ORIGINAL==
+
+export function /*[#|*/foo/*|]*/() {
+    return fetch('https://typescriptlang.org').then(s => console.log(s));
+}
+
+// ==ASYNC FUNCTION::Convert to async function==
+
+export async function foo() {
+    const s = await fetch('https://typescriptlang.org');
+    return console.log(s);
+}