Allow JS constructor function to return non-void

src/compiler/checker.ts

@@ -15835,7 +15835,7 @@ namespace ts {
             const callSignatures = getSignaturesOfType(expressionType, SignatureKind.Call);
             if (callSignatures.length) {
                 const signature = resolveCall(node, callSignatures, candidatesOutArray);
-                if (getReturnTypeOfSignature(signature) !== voidType) {
+                if (!isInJavaScriptFile(signature.declaration) && getReturnTypeOfSignature(signature) !== voidType) {
                     error(node, Diagnostics.Only_a_void_function_can_be_called_with_the_new_keyword);
                 }
                 if (getThisTypeOfSignature(signature) === voidType) {

tests/baselines/reference/jsConstructorFunction.symbols

@@ -0,0 +1,56 @@
+=== tests/cases/compiler/index.js ===
+var Person = function (firstNameOrPojo, lastName) {
+>Person : Symbol(Person, Decl(index.js, 0, 3))
+>firstNameOrPojo : Symbol(firstNameOrPojo, Decl(index.js, 0, 23))
+>lastName : Symbol(lastName, Decl(index.js, 0, 39))
+
+    if (typeof firstNameOrPojo === "string") {
+>firstNameOrPojo : Symbol(firstNameOrPojo, Decl(index.js, 0, 23))
+
+        this.firstName = firstNameOrPojo;
+>firstName : Symbol(Person.firstName, Decl(index.js, 2, 46))
+>firstNameOrPojo : Symbol(firstNameOrPojo, Decl(index.js, 0, 23))
+
+        this.lastName = lastName;
+>lastName : Symbol(Person.lastName, Decl(index.js, 3, 41))
+>lastName : Symbol(lastName, Decl(index.js, 0, 39))
+
+    } else {
+        return new Person(firstNameOrPojo.firstName, firstNameOrPojo.lastName);
+>Person : Symbol(Person, Decl(index.js, 0, 3))
+>firstNameOrPojo : Symbol(firstNameOrPojo, Decl(index.js, 0, 23))
+>firstNameOrPojo : Symbol(firstNameOrPojo, Decl(index.js, 0, 23))
+    }
+};
+
+Person.prototype.greet = function greet() {
+>Person.prototype : Symbol(Person.greet, Decl(index.js, 8, 2))
+>Person : Symbol(Person, Decl(index.js, 0, 3))
+>prototype : Symbol(Function.prototype, Decl(lib.es5.d.ts, --, --))
+>greet : Symbol(Person.greet, Decl(index.js, 8, 2))
+>greet : Symbol(greet, Decl(index.js, 10, 24))
+
+    return `Hello, I am ${this.firstName} ${this.lastName}.`;
+>this.firstName : Symbol(Person.firstName, Decl(index.js, 2, 46))
+>this : Symbol(Person, Decl(index.js, 0, 12))
+>firstName : Symbol(Person.firstName, Decl(index.js, 2, 46))
+>this.lastName : Symbol(Person.lastName, Decl(index.js, 3, 41))
+>this : Symbol(Person, Decl(index.js, 0, 12))
+>lastName : Symbol(Person.lastName, Decl(index.js, 3, 41))
+
+};
+
+var fred = new Person({ firstName: "Fred", lastName: "Flintstone" });
+>fred : Symbol(fred, Decl(index.js, 14, 3))
+>Person : Symbol(Person, Decl(index.js, 0, 3))
+>firstName : Symbol(firstName, Decl(index.js, 14, 23))
+>lastName : Symbol(lastName, Decl(index.js, 14, 42))
+
+console.log(fred.greet());
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>fred.greet : Symbol(Person.greet, Decl(index.js, 8, 2))
+>fred : Symbol(fred, Decl(index.js, 14, 3))
+>greet : Symbol(Person.greet, Decl(index.js, 8, 2))
+

tests/baselines/reference/jsConstructorFunction.types

@@ -0,0 +1,81 @@
+=== tests/cases/compiler/index.js ===
+var Person = function (firstNameOrPojo, lastName) {
+>Person : (firstNameOrPojo: any, lastName: any) => any
+>function (firstNameOrPojo, lastName) {    if (typeof firstNameOrPojo === "string") {        this.firstName = firstNameOrPojo;        this.lastName = lastName;    } else {        return new Person(firstNameOrPojo.firstName, firstNameOrPojo.lastName);    }} : (firstNameOrPojo: any, lastName: any) => any
+>firstNameOrPojo : any
+>lastName : any
+
+    if (typeof firstNameOrPojo === "string") {
+>typeof firstNameOrPojo === "string" : boolean
+>typeof firstNameOrPojo : "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>firstNameOrPojo : any
+>"string" : "string"
+
+        this.firstName = firstNameOrPojo;
+>this.firstName = firstNameOrPojo : string
+>this.firstName : any
+>this : any
+>firstName : any
+>firstNameOrPojo : string
+
+        this.lastName = lastName;
+>this.lastName = lastName : any
+>this.lastName : any
+>this : any
+>lastName : any
+>lastName : any
+
+    } else {
+        return new Person(firstNameOrPojo.firstName, firstNameOrPojo.lastName);
+>new Person(firstNameOrPojo.firstName, firstNameOrPojo.lastName) : { firstName: string; lastName: any; greet: () => string; }
+>Person : (firstNameOrPojo: any, lastName: any) => any
+>firstNameOrPojo.firstName : any
+>firstNameOrPojo : any
+>firstName : any
+>firstNameOrPojo.lastName : any
+>firstNameOrPojo : any
+>lastName : any
+    }
+};
+
+Person.prototype.greet = function greet() {
+>Person.prototype.greet = function greet() {    return `Hello, I am ${this.firstName} ${this.lastName}.`;} : () => string
+>Person.prototype.greet : any
+>Person.prototype : any
+>Person : (firstNameOrPojo: any, lastName: any) => any
+>prototype : any
+>greet : any
+>function greet() {    return `Hello, I am ${this.firstName} ${this.lastName}.`;} : () => string
+>greet : () => string
+
+    return `Hello, I am ${this.firstName} ${this.lastName}.`;
+>`Hello, I am ${this.firstName} ${this.lastName}.` : string
+>this.firstName : string
+>this : { firstName: string; lastName: any; greet: () => string; }
+>firstName : string
+>this.lastName : any
+>this : { firstName: string; lastName: any; greet: () => string; }
+>lastName : any
+
+};
+
+var fred = new Person({ firstName: "Fred", lastName: "Flintstone" });
+>fred : { firstName: string; lastName: any; greet: () => string; }
+>new Person({ firstName: "Fred", lastName: "Flintstone" }) : { firstName: string; lastName: any; greet: () => string; }
+>Person : (firstNameOrPojo: any, lastName: any) => any
+>{ firstName: "Fred", lastName: "Flintstone" } : { firstName: string; lastName: string; }
+>firstName : string
+>"Fred" : "Fred"
+>lastName : string
+>"Flintstone" : "Flintstone"
+
+console.log(fred.greet());
+>console.log(fred.greet()) : void
+>console.log : (message?: any, ...optionalParams: any[]) => void
+>console : Console
+>log : (message?: any, ...optionalParams: any[]) => void
+>fred.greet() : string
+>fred.greet : () => string
+>fred : { firstName: string; lastName: any; greet: () => string; }
+>greet : () => string
+

tests/cases/compiler/jsConstructorFunction.ts

@@ -0,0 +1,22 @@
+// @allowJs: true
+// @checkJs: true
+// @noEmit: true
+// @filename: index.js
+// @lib: es5, dom
+var Person = function (firstNameOrPojo, lastName) {
+
+    if (typeof firstNameOrPojo === "string") {
+        this.firstName = firstNameOrPojo;
+        this.lastName = lastName;
+    } else {
+        return new Person(firstNameOrPojo.firstName, firstNameOrPojo.lastName);
+    }
+};
+
+Person.prototype.greet = function greet() {
+    return `Hello, I am ${this.firstName} ${this.lastName}.`;
+};
+
+var fred = new Person({ firstName: "Fred", lastName: "Flintstone" });
+
+console.log(fred.greet());