diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 24ca7da87c..86193a5807 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -3993,14 +3993,15 @@ namespace ts {
             return getClassExtendsHeritageClauseElement(<ClassLikeDeclaration>type.symbol.valueDeclaration);
         }
 
-        function getConstructorsForTypeArguments(type: Type, typeArgumentNodes: TypeNode[]): Signature[] {
+        function getConstructorsForTypeArguments(type: Type, typeArgumentNodes: TypeNode[], location: Node): Signature[] {
             const typeArgCount = length(typeArgumentNodes);
+            const isJavaScript = isInJavaScriptFile(location);
             return filter(getSignaturesOfType(type, SignatureKind.Construct),
-                sig => typeArgCount >= getMinTypeArgumentCount(sig.typeParameters) && typeArgCount <= length(sig.typeParameters));
+                sig => (isJavaScript || typeArgCount >= getMinTypeArgumentCount(sig.typeParameters)) && typeArgCount <= length(sig.typeParameters));
         }
 
-        function getInstantiatedConstructorsForTypeArguments(type: Type, typeArgumentNodes: TypeNode[]): Signature[] {
-            let signatures = getConstructorsForTypeArguments(type, typeArgumentNodes);
+        function getInstantiatedConstructorsForTypeArguments(type: Type, typeArgumentNodes: TypeNode[], location: Node): Signature[] {
+            let signatures = getConstructorsForTypeArguments(type, typeArgumentNodes, location);
             if (typeArgumentNodes) {
                 const typeArguments = map(typeArgumentNodes, getTypeFromTypeNode);
                 signatures = map(signatures, sig => getSignatureInstantiation(sig, typeArguments));
@@ -4083,7 +4084,7 @@ namespace ts {
                 // The class derives from a "class-like" constructor function, check that we have at least one construct signature
                 // with a matching number of type parameters and use the return type of the first instantiated signature. Elsewhere
                 // we check that all instantiated signatures return the same type.
-                const constructors = getInstantiatedConstructorsForTypeArguments(baseConstructorType, baseTypeNode.typeArguments);
+                const constructors = getInstantiatedConstructorsForTypeArguments(baseConstructorType, baseTypeNode.typeArguments, baseTypeNode);
                 if (!constructors.length) {
                     error(baseTypeNode.expression, Diagnostics.No_base_constructor_has_the_specified_number_of_type_arguments);
                     return;
@@ -4593,14 +4594,15 @@ namespace ts {
                 return [createSignature(undefined, classType.localTypeParameters, undefined, emptyArray, classType, /*typePredicate*/ undefined, 0, /*hasRestParameter*/ false, /*hasLiteralTypes*/ false)];
             }
             const baseTypeNode = getBaseTypeNodeOfClass(classType);
+            const isJavaScript = isInJavaScriptFile(baseTypeNode);
             const typeArguments = map(baseTypeNode.typeArguments, getTypeFromTypeNode);
             const typeArgCount = length(typeArguments);
             const result: Signature[] = [];
             for (const baseSig of baseSignatures) {
                 const minTypeArgumentCount = getMinTypeArgumentCount(baseSig.typeParameters);
                 const typeParamCount = length(baseSig.typeParameters);
-                if (typeArgCount >= minTypeArgumentCount && typeArgCount <= typeParamCount) {
-                    const sig = typeParamCount ? createSignatureInstantiation(baseSig, fillMissingTypeArguments(typeArguments, baseSig.typeParameters, minTypeArgumentCount)) : cloneSignature(baseSig);
+                if ((isJavaScript || typeArgCount >= minTypeArgumentCount) && typeArgCount <= typeParamCount) {
+                    const sig = typeParamCount ? createSignatureInstantiation(baseSig, fillMissingTypeArguments(typeArguments, baseSig.typeParameters, minTypeArgumentCount, baseTypeNode)) : cloneSignature(baseSig);
                     sig.typeParameters = classType.localTypeParameters;
                     sig.resolvedReturnType = classType;
                     result.push(sig);
@@ -5433,11 +5435,12 @@ namespace ts {
          * @param typeParameters The requested type parameters.
          * @param minTypeArgumentCount The minimum number of required type arguments.
          */
-        function fillMissingTypeArguments(typeArguments: Type[] | undefined, typeParameters: TypeParameter[] | undefined, minTypeArgumentCount: number) {
+        function fillMissingTypeArguments(typeArguments: Type[] | undefined, typeParameters: TypeParameter[] | undefined, minTypeArgumentCount: number, location?: Node) {
             const numTypeParameters = length(typeParameters);
             if (numTypeParameters) {
                 const numTypeArguments = length(typeArguments);
-                if (numTypeArguments >= minTypeArgumentCount && numTypeArguments <= numTypeParameters) {
+                const isJavaScript = isInJavaScriptFile(location);
+                if ((isJavaScript || numTypeArguments >= minTypeArgumentCount) && numTypeArguments <= numTypeParameters) {
                     if (!typeArguments) {
                         typeArguments = [];
                     }
@@ -5446,12 +5449,12 @@ namespace ts {
                     // If a type parameter does not have a default type, or if the default type
                     // is a forward reference, the empty object type is used.
                     for (let i = numTypeArguments; i < numTypeParameters; i++) {
-                        typeArguments[i] = emptyObjectType;
+                        typeArguments[i] = isJavaScript ? anyType : emptyObjectType;
                     }
                     for (let i = numTypeArguments; i < numTypeParameters; i++) {
                         const mapper = createTypeMapper(typeParameters, typeArguments);
                         const defaultType = getDefaultFromTypeParameter(typeParameters[i]);
-                        typeArguments[i] = defaultType ? instantiateType(defaultType, mapper) : emptyObjectType;
+                        typeArguments[i] = defaultType ? instantiateType(defaultType, mapper) : isJavaScript ? anyType : emptyObjectType;
                     }
                 }
             }
@@ -5825,7 +5828,7 @@ namespace ts {
             if (typeParameters) {
                 const numTypeArguments = length(node.typeArguments);
                 const minTypeArgumentCount = getMinTypeArgumentCount(typeParameters);
-                if (numTypeArguments < minTypeArgumentCount || numTypeArguments > typeParameters.length) {
+                if (!isInJavaScriptFile(node) && (numTypeArguments < minTypeArgumentCount || numTypeArguments > typeParameters.length)) {
                     error(node,
                         minTypeArgumentCount === typeParameters.length
                             ? Diagnostics.Generic_type_0_requires_1_type_argument_s
@@ -5838,7 +5841,7 @@ namespace ts {
                 // In a type reference, the outer type parameters of the referenced class or interface are automatically
                 // supplied as type arguments and the type reference only specifies arguments for the local type parameters
                 // of the class or interface.
-                const typeArguments = concatenate(type.outerTypeParameters, fillMissingTypeArguments(map(node.typeArguments, getTypeFromTypeNode), typeParameters, minTypeArgumentCount));
+                const typeArguments = concatenate(type.outerTypeParameters, fillMissingTypeArguments(map(node.typeArguments, getTypeFromTypeNode), typeParameters, minTypeArgumentCount, node));
                 return createTypeReference(<GenericType>type, typeArguments);
             }
             if (node.typeArguments) {
@@ -14481,7 +14484,7 @@ namespace ts {
                     // with the type arguments specified in the extends clause.
                     const baseTypeNode = getClassExtendsHeritageClauseElement(getContainingClass(node));
                     if (baseTypeNode) {
-                        const baseConstructors = getInstantiatedConstructorsForTypeArguments(superType, baseTypeNode.typeArguments);
+                        const baseConstructors = getInstantiatedConstructorsForTypeArguments(superType, baseTypeNode.typeArguments, baseTypeNode);
                         return resolveCall(node, baseConstructors, candidatesOutArray);
                     }
                 }
@@ -19595,7 +19598,7 @@ namespace ts {
                     checkSourceElement(baseTypeNode.expression);
                     if (baseTypeNode.typeArguments) {
                         forEach(baseTypeNode.typeArguments, checkSourceElement);
-                        for (const constructor of getConstructorsForTypeArguments(staticBaseType, baseTypeNode.typeArguments)) {
+                        for (const constructor of getConstructorsForTypeArguments(staticBaseType, baseTypeNode.typeArguments, baseTypeNode)) {
                             if (!checkTypeArgumentConstraints(constructor.typeParameters, baseTypeNode.typeArguments)) {
                                 break;
                             }
@@ -19613,7 +19616,7 @@ namespace ts {
                         // that all instantiated base constructor signatures return the same type. We can simply compare the type
                         // references (as opposed to checking the structure of the types) because elsewhere we have already checked
                         // that the base type is a class or interface type (and not, for example, an anonymous object type).
-                        const constructors = getInstantiatedConstructorsForTypeArguments(staticBaseType, baseTypeNode.typeArguments);
+                        const constructors = getInstantiatedConstructorsForTypeArguments(staticBaseType, baseTypeNode.typeArguments, baseTypeNode);
                         if (forEach(constructors, sig => getReturnTypeOfSignature(sig) !== baseType)) {
                             error(baseTypeNode.expression, Diagnostics.Base_constructors_must_all_have_the_same_return_type);
                         }
diff --git a/tests/baselines/reference/genericDefaultsJs.symbols b/tests/baselines/reference/genericDefaultsJs.symbols
new file mode 100644
index 0000000000..633e4cc8fe
--- /dev/null
+++ b/tests/baselines/reference/genericDefaultsJs.symbols
@@ -0,0 +1,226 @@
+=== tests/cases/compiler/decls.d.ts ===
+declare function f0<T>(x?: T): T;
+>f0 : Symbol(f0, Decl(decls.d.ts, 0, 0))
+>T : Symbol(T, Decl(decls.d.ts, 0, 20))
+>x : Symbol(x, Decl(decls.d.ts, 0, 23))
+>T : Symbol(T, Decl(decls.d.ts, 0, 20))
+>T : Symbol(T, Decl(decls.d.ts, 0, 20))
+
+declare function f1<T, U = number>(x?: T): [T, U];
+>f1 : Symbol(f1, Decl(decls.d.ts, 0, 33))
+>T : Symbol(T, Decl(decls.d.ts, 1, 20))
+>U : Symbol(U, Decl(decls.d.ts, 1, 22))
+>x : Symbol(x, Decl(decls.d.ts, 1, 35))
+>T : Symbol(T, Decl(decls.d.ts, 1, 20))
+>T : Symbol(T, Decl(decls.d.ts, 1, 20))
+>U : Symbol(U, Decl(decls.d.ts, 1, 22))
+
+declare class C0<T> {
+>C0 : Symbol(C0, Decl(decls.d.ts, 1, 50))
+>T : Symbol(T, Decl(decls.d.ts, 2, 17))
+
+    y: T;
+>y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+>T : Symbol(T, Decl(decls.d.ts, 2, 17))
+
+    constructor(x?: T);
+>x : Symbol(x, Decl(decls.d.ts, 4, 16))
+>T : Symbol(T, Decl(decls.d.ts, 2, 17))
+}
+declare class C1<T, U = number> {
+>C1 : Symbol(C1, Decl(decls.d.ts, 5, 1))
+>T : Symbol(T, Decl(decls.d.ts, 6, 17))
+>U : Symbol(U, Decl(decls.d.ts, 6, 19))
+
+    y: [T, U];
+>y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+>T : Symbol(T, Decl(decls.d.ts, 6, 17))
+>U : Symbol(U, Decl(decls.d.ts, 6, 19))
+
+    constructor(x?: T);
+>x : Symbol(x, Decl(decls.d.ts, 8, 16))
+>T : Symbol(T, Decl(decls.d.ts, 6, 17))
+}
+=== tests/cases/compiler/main.js ===
+const f0_v0 = f0();
+>f0_v0 : Symbol(f0_v0, Decl(main.js, 0, 5))
+>f0 : Symbol(f0, Decl(decls.d.ts, 0, 0))
+
+const f0_v1 = f0(1);
+>f0_v1 : Symbol(f0_v1, Decl(main.js, 1, 5))
+>f0 : Symbol(f0, Decl(decls.d.ts, 0, 0))
+
+const f1_c0 = f1();
+>f1_c0 : Symbol(f1_c0, Decl(main.js, 3, 5))
+>f1 : Symbol(f1, Decl(decls.d.ts, 0, 33))
+
+const f1_c1 = f1(1);
+>f1_c1 : Symbol(f1_c1, Decl(main.js, 4, 5))
+>f1 : Symbol(f1, Decl(decls.d.ts, 0, 33))
+
+const C0_v0 = new C0();
+>C0_v0 : Symbol(C0_v0, Decl(main.js, 6, 5))
+>C0 : Symbol(C0, Decl(decls.d.ts, 1, 50))
+
+const C0_v0_y = C0_v0.y;
+>C0_v0_y : Symbol(C0_v0_y, Decl(main.js, 7, 5))
+>C0_v0.y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+>C0_v0 : Symbol(C0_v0, Decl(main.js, 6, 5))
+>y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+
+const C0_v1 = new C0(1);
+>C0_v1 : Symbol(C0_v1, Decl(main.js, 9, 5))
+>C0 : Symbol(C0, Decl(decls.d.ts, 1, 50))
+
+const C0_v1_y = C0_v1.y;
+>C0_v1_y : Symbol(C0_v1_y, Decl(main.js, 10, 5))
+>C0_v1.y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+>C0_v1 : Symbol(C0_v1, Decl(main.js, 9, 5))
+>y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+
+const C1_v0 = new C1();
+>C1_v0 : Symbol(C1_v0, Decl(main.js, 12, 5))
+>C1 : Symbol(C1, Decl(decls.d.ts, 5, 1))
+
+const C1_v0_y = C1_v0.y;
+>C1_v0_y : Symbol(C1_v0_y, Decl(main.js, 13, 5))
+>C1_v0.y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+>C1_v0 : Symbol(C1_v0, Decl(main.js, 12, 5))
+>y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+
+const C1_v1 = new C1(1);
+>C1_v1 : Symbol(C1_v1, Decl(main.js, 15, 5))
+>C1 : Symbol(C1, Decl(decls.d.ts, 5, 1))
+
+const C1_v1_y = C1_v1.y;
+>C1_v1_y : Symbol(C1_v1_y, Decl(main.js, 16, 5))
+>C1_v1.y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+>C1_v1 : Symbol(C1_v1, Decl(main.js, 15, 5))
+>y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+
+class C0_B0 extends C0 {}
+>C0_B0 : Symbol(C0_B0, Decl(main.js, 16, 24))
+>C0 : Symbol(C0, Decl(decls.d.ts, 1, 50))
+
+class C0_B1 extends C0 {
+>C0_B1 : Symbol(C0_B1, Decl(main.js, 18, 25))
+>C0 : Symbol(C0, Decl(decls.d.ts, 1, 50))
+
+    constructor() {
+        super();
+>super : Symbol(C0, Decl(decls.d.ts, 1, 50))
+    }
+}
+class C0_B2 extends C0 {
+>C0_B2 : Symbol(C0_B2, Decl(main.js, 23, 1))
+>C0 : Symbol(C0, Decl(decls.d.ts, 1, 50))
+
+    constructor() {
+        super(1);
+>super : Symbol(C0, Decl(decls.d.ts, 1, 50))
+    }
+}
+
+const C0_B0_v0 = new C0_B0();
+>C0_B0_v0 : Symbol(C0_B0_v0, Decl(main.js, 30, 5))
+>C0_B0 : Symbol(C0_B0, Decl(main.js, 16, 24))
+
+const C0_B0_v0_y = C0_B0_v0.y;
+>C0_B0_v0_y : Symbol(C0_B0_v0_y, Decl(main.js, 31, 5))
+>C0_B0_v0.y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+>C0_B0_v0 : Symbol(C0_B0_v0, Decl(main.js, 30, 5))
+>y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+
+const C0_B0_v1 = new C0_B0(1);
+>C0_B0_v1 : Symbol(C0_B0_v1, Decl(main.js, 33, 5))
+>C0_B0 : Symbol(C0_B0, Decl(main.js, 16, 24))
+
+const C0_B0_v1_y = C0_B0_v1.y;
+>C0_B0_v1_y : Symbol(C0_B0_v1_y, Decl(main.js, 34, 5))
+>C0_B0_v1.y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+>C0_B0_v1 : Symbol(C0_B0_v1, Decl(main.js, 33, 5))
+>y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+
+const C0_B1_v0 = new C0_B1();
+>C0_B1_v0 : Symbol(C0_B1_v0, Decl(main.js, 36, 5))
+>C0_B1 : Symbol(C0_B1, Decl(main.js, 18, 25))
+
+const C0_B1_v0_y = C0_B1_v0.y;
+>C0_B1_v0_y : Symbol(C0_B1_v0_y, Decl(main.js, 37, 5))
+>C0_B1_v0.y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+>C0_B1_v0 : Symbol(C0_B1_v0, Decl(main.js, 36, 5))
+>y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+
+const C0_B2_v0 = new C0_B2();
+>C0_B2_v0 : Symbol(C0_B2_v0, Decl(main.js, 39, 5))
+>C0_B2 : Symbol(C0_B2, Decl(main.js, 23, 1))
+
+const C0_B2_v0_y = C0_B2_v0.y;
+>C0_B2_v0_y : Symbol(C0_B2_v0_y, Decl(main.js, 40, 5))
+>C0_B2_v0.y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+>C0_B2_v0 : Symbol(C0_B2_v0, Decl(main.js, 39, 5))
+>y : Symbol(C0.y, Decl(decls.d.ts, 2, 21))
+
+class C1_B0 extends C1 {}
+>C1_B0 : Symbol(C1_B0, Decl(main.js, 40, 30))
+>C1 : Symbol(C1, Decl(decls.d.ts, 5, 1))
+
+class C1_B1 extends C1 {
+>C1_B1 : Symbol(C1_B1, Decl(main.js, 42, 25))
+>C1 : Symbol(C1, Decl(decls.d.ts, 5, 1))
+
+    constructor() {
+        super();
+>super : Symbol(C1, Decl(decls.d.ts, 5, 1))
+    }
+}
+class C1_B2 extends C1 {
+>C1_B2 : Symbol(C1_B2, Decl(main.js, 47, 1))
+>C1 : Symbol(C1, Decl(decls.d.ts, 5, 1))
+
+    constructor() {
+        super(1);
+>super : Symbol(C1, Decl(decls.d.ts, 5, 1))
+    }
+}
+
+const C1_B0_v0 = new C1_B0();
+>C1_B0_v0 : Symbol(C1_B0_v0, Decl(main.js, 54, 5))
+>C1_B0 : Symbol(C1_B0, Decl(main.js, 40, 30))
+
+const C1_B0_v0_y = C1_B0_v0.y;
+>C1_B0_v0_y : Symbol(C1_B0_v0_y, Decl(main.js, 55, 5))
+>C1_B0_v0.y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+>C1_B0_v0 : Symbol(C1_B0_v0, Decl(main.js, 54, 5))
+>y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+
+const C1_B0_v1 = new C1_B0(1);
+>C1_B0_v1 : Symbol(C1_B0_v1, Decl(main.js, 57, 5))
+>C1_B0 : Symbol(C1_B0, Decl(main.js, 40, 30))
+
+const C1_B0_v1_y = C1_B0_v1.y;
+>C1_B0_v1_y : Symbol(C1_B0_v1_y, Decl(main.js, 58, 5))
+>C1_B0_v1.y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+>C1_B0_v1 : Symbol(C1_B0_v1, Decl(main.js, 57, 5))
+>y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+
+const C1_B1_v0 = new C1_B1();
+>C1_B1_v0 : Symbol(C1_B1_v0, Decl(main.js, 60, 5))
+>C1_B1 : Symbol(C1_B1, Decl(main.js, 42, 25))
+
+const C1_B1_v0_y = C1_B1_v0.y;
+>C1_B1_v0_y : Symbol(C1_B1_v0_y, Decl(main.js, 61, 5))
+>C1_B1_v0.y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+>C1_B1_v0 : Symbol(C1_B1_v0, Decl(main.js, 60, 5))
+>y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+
+const C1_B2_v0 = new C1_B2();
+>C1_B2_v0 : Symbol(C1_B2_v0, Decl(main.js, 63, 5))
+>C1_B2 : Symbol(C1_B2, Decl(main.js, 47, 1))
+
+const C1_B2_v0_y = C1_B2_v0.y;
+>C1_B2_v0_y : Symbol(C1_B2_v0_y, Decl(main.js, 64, 5))
+>C1_B2_v0.y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+>C1_B2_v0 : Symbol(C1_B2_v0, Decl(main.js, 63, 5))
+>y : Symbol(C1.y, Decl(decls.d.ts, 6, 33))
+
diff --git a/tests/baselines/reference/genericDefaultsJs.types b/tests/baselines/reference/genericDefaultsJs.types
new file mode 100644
index 0000000000..3275e757d7
--- /dev/null
+++ b/tests/baselines/reference/genericDefaultsJs.types
@@ -0,0 +1,254 @@
+=== tests/cases/compiler/decls.d.ts ===
+declare function f0<T>(x?: T): T;
+>f0 : <T>(x?: T) => T
+>T : T
+>x : T
+>T : T
+>T : T
+
+declare function f1<T, U = number>(x?: T): [T, U];
+>f1 : <T, U = number>(x?: T) => [T, U]
+>T : T
+>U : U
+>x : T
+>T : T
+>T : T
+>U : U
+
+declare class C0<T> {
+>C0 : C0<T>
+>T : T
+
+    y: T;
+>y : T
+>T : T
+
+    constructor(x?: T);
+>x : T
+>T : T
+}
+declare class C1<T, U = number> {
+>C1 : C1<T, U>
+>T : T
+>U : U
+
+    y: [T, U];
+>y : [T, U]
+>T : T
+>U : U
+
+    constructor(x?: T);
+>x : T
+>T : T
+}
+=== tests/cases/compiler/main.js ===
+const f0_v0 = f0();
+>f0_v0 : any
+>f0() : any
+>f0 : <T>(x?: T) => T
+
+const f0_v1 = f0(1);
+>f0_v1 : 1
+>f0(1) : 1
+>f0 : <T>(x?: T) => T
+>1 : 1
+
+const f1_c0 = f1();
+>f1_c0 : [any, number]
+>f1() : [any, number]
+>f1 : <T, U = number>(x?: T) => [T, U]
+
+const f1_c1 = f1(1);
+>f1_c1 : [number, number]
+>f1(1) : [number, number]
+>f1 : <T, U = number>(x?: T) => [T, U]
+>1 : 1
+
+const C0_v0 = new C0();
+>C0_v0 : C0<any>
+>new C0() : C0<any>
+>C0 : typeof C0
+
+const C0_v0_y = C0_v0.y;
+>C0_v0_y : any
+>C0_v0.y : any
+>C0_v0 : C0<any>
+>y : any
+
+const C0_v1 = new C0(1);
+>C0_v1 : C0<number>
+>new C0(1) : C0<number>
+>C0 : typeof C0
+>1 : 1
+
+const C0_v1_y = C0_v1.y;
+>C0_v1_y : number
+>C0_v1.y : number
+>C0_v1 : C0<number>
+>y : number
+
+const C1_v0 = new C1();
+>C1_v0 : C1<any, number>
+>new C1() : C1<any, number>
+>C1 : typeof C1
+
+const C1_v0_y = C1_v0.y;
+>C1_v0_y : [any, number]
+>C1_v0.y : [any, number]
+>C1_v0 : C1<any, number>
+>y : [any, number]
+
+const C1_v1 = new C1(1);
+>C1_v1 : C1<number, number>
+>new C1(1) : C1<number, number>
+>C1 : typeof C1
+>1 : 1
+
+const C1_v1_y = C1_v1.y;
+>C1_v1_y : [number, number]
+>C1_v1.y : [number, number]
+>C1_v1 : C1<number, number>
+>y : [number, number]
+
+class C0_B0 extends C0 {}
+>C0_B0 : C0_B0
+>C0 : C0<any>
+
+class C0_B1 extends C0 {
+>C0_B1 : C0_B1
+>C0 : C0<any>
+
+    constructor() {
+        super();
+>super() : void
+>super : typeof C0
+    }
+}
+class C0_B2 extends C0 {
+>C0_B2 : C0_B2
+>C0 : C0<any>
+
+    constructor() {
+        super(1);
+>super(1) : void
+>super : typeof C0
+>1 : 1
+    }
+}
+
+const C0_B0_v0 = new C0_B0();
+>C0_B0_v0 : C0_B0
+>new C0_B0() : C0_B0
+>C0_B0 : typeof C0_B0
+
+const C0_B0_v0_y = C0_B0_v0.y;
+>C0_B0_v0_y : any
+>C0_B0_v0.y : any
+>C0_B0_v0 : C0_B0
+>y : any
+
+const C0_B0_v1 = new C0_B0(1);
+>C0_B0_v1 : C0_B0
+>new C0_B0(1) : C0_B0
+>C0_B0 : typeof C0_B0
+>1 : 1
+
+const C0_B0_v1_y = C0_B0_v1.y;
+>C0_B0_v1_y : any
+>C0_B0_v1.y : any
+>C0_B0_v1 : C0_B0
+>y : any
+
+const C0_B1_v0 = new C0_B1();
+>C0_B1_v0 : C0_B1
+>new C0_B1() : C0_B1
+>C0_B1 : typeof C0_B1
+
+const C0_B1_v0_y = C0_B1_v0.y;
+>C0_B1_v0_y : any
+>C0_B1_v0.y : any
+>C0_B1_v0 : C0_B1
+>y : any
+
+const C0_B2_v0 = new C0_B2();
+>C0_B2_v0 : C0_B2
+>new C0_B2() : C0_B2
+>C0_B2 : typeof C0_B2
+
+const C0_B2_v0_y = C0_B2_v0.y;
+>C0_B2_v0_y : any
+>C0_B2_v0.y : any
+>C0_B2_v0 : C0_B2
+>y : any
+
+class C1_B0 extends C1 {}
+>C1_B0 : C1_B0
+>C1 : C1<any, number>
+
+class C1_B1 extends C1 {
+>C1_B1 : C1_B1
+>C1 : C1<any, number>
+
+    constructor() {
+        super();
+>super() : void
+>super : typeof C1
+    }
+}
+class C1_B2 extends C1 {
+>C1_B2 : C1_B2
+>C1 : C1<any, number>
+
+    constructor() {
+        super(1);
+>super(1) : void
+>super : typeof C1
+>1 : 1
+    }
+}
+
+const C1_B0_v0 = new C1_B0();
+>C1_B0_v0 : C1_B0
+>new C1_B0() : C1_B0
+>C1_B0 : typeof C1_B0
+
+const C1_B0_v0_y = C1_B0_v0.y;
+>C1_B0_v0_y : [any, number]
+>C1_B0_v0.y : [any, number]
+>C1_B0_v0 : C1_B0
+>y : [any, number]
+
+const C1_B0_v1 = new C1_B0(1);
+>C1_B0_v1 : C1_B0
+>new C1_B0(1) : C1_B0
+>C1_B0 : typeof C1_B0
+>1 : 1
+
+const C1_B0_v1_y = C1_B0_v1.y;
+>C1_B0_v1_y : [any, number]
+>C1_B0_v1.y : [any, number]
+>C1_B0_v1 : C1_B0
+>y : [any, number]
+
+const C1_B1_v0 = new C1_B1();
+>C1_B1_v0 : C1_B1
+>new C1_B1() : C1_B1
+>C1_B1 : typeof C1_B1
+
+const C1_B1_v0_y = C1_B1_v0.y;
+>C1_B1_v0_y : [any, number]
+>C1_B1_v0.y : [any, number]
+>C1_B1_v0 : C1_B1
+>y : [any, number]
+
+const C1_B2_v0 = new C1_B2();
+>C1_B2_v0 : C1_B2
+>new C1_B2() : C1_B2
+>C1_B2 : typeof C1_B2
+
+const C1_B2_v0_y = C1_B2_v0.y;
+>C1_B2_v0_y : [any, number]
+>C1_B2_v0.y : [any, number]
+>C1_B2_v0 : C1_B2
+>y : [any, number]
+
diff --git a/tests/cases/compiler/genericDefaultsJs.ts b/tests/cases/compiler/genericDefaultsJs.ts
new file mode 100644
index 0000000000..6aa3640d96
--- /dev/null
+++ b/tests/cases/compiler/genericDefaultsJs.ts
@@ -0,0 +1,80 @@
+// @checkJs: true
+// @allowJs: true
+// @noEmit: true
+// @filename: decls.d.ts
+declare function f0<T>(x?: T): T;
+declare function f1<T, U = number>(x?: T): [T, U];
+declare class C0<T> {
+    y: T;
+    constructor(x?: T);
+}
+declare class C1<T, U = number> {
+    y: [T, U];
+    constructor(x?: T);
+}
+// @filename: main.js
+const f0_v0 = f0();
+const f0_v1 = f0(1);
+
+const f1_c0 = f1();
+const f1_c1 = f1(1);
+
+const C0_v0 = new C0();
+const C0_v0_y = C0_v0.y;
+
+const C0_v1 = new C0(1);
+const C0_v1_y = C0_v1.y;
+
+const C1_v0 = new C1();
+const C1_v0_y = C1_v0.y;
+
+const C1_v1 = new C1(1);
+const C1_v1_y = C1_v1.y;
+
+class C0_B0 extends C0 {}
+class C0_B1 extends C0 {
+    constructor() {
+        super();
+    }
+}
+class C0_B2 extends C0 {
+    constructor() {
+        super(1);
+    }
+}
+
+const C0_B0_v0 = new C0_B0();
+const C0_B0_v0_y = C0_B0_v0.y;
+
+const C0_B0_v1 = new C0_B0(1);
+const C0_B0_v1_y = C0_B0_v1.y;
+
+const C0_B1_v0 = new C0_B1();
+const C0_B1_v0_y = C0_B1_v0.y;
+
+const C0_B2_v0 = new C0_B2();
+const C0_B2_v0_y = C0_B2_v0.y;
+
+class C1_B0 extends C1 {}
+class C1_B1 extends C1 {
+    constructor() {
+        super();
+    }
+}
+class C1_B2 extends C1 {
+    constructor() {
+        super(1);
+    }
+}
+
+const C1_B0_v0 = new C1_B0();
+const C1_B0_v0_y = C1_B0_v0.y;
+
+const C1_B0_v1 = new C1_B0(1);
+const C1_B0_v1_y = C1_B0_v1.y;
+
+const C1_B1_v0 = new C1_B1();
+const C1_B1_v0_y = C1_B1_v0.y;
+
+const C1_B2_v0 = new C1_B2();
+const C1_B2_v0_y = C1_B2_v0.y;