Fix legacy factory functions for non‑default new.target values

Author: ExE-Boss

README.md

@@ -282,11 +282,11 @@ Creates a new instance of the wrapper class and corresponding implementation cla
 
 This is useful inside implementation class files, where it is easiest to only deal with impls, not wrappers.
 
-#### `new(globalObject)`
+#### `new(globalObject [ , newTarget ])`
 
 Creates a new instance of the wrapper class and corresponding implementation class, but without invoking the implementation class constructor logic. Then returns the implementation class.
 
-This corresponds to the [Web IDL "new" algorithm](https://heycam.github.io/webidl/#new), and is useful when implementing specifications that initialize objects in different ways than their constructors do.
+This corresponds to the [WebIDL "create a new object implementing the interface"](https://heycam.github.io/webidl/#new) and ["internally create a new object implementing the interface"](https://heycam.github.io/webidl/#internally-create-a-new-object-implementing-the-interface) algorithms, and is useful when implementing specifications that initialize objects in different ways than their constructors do.
 
 #### `setup(obj, globalObject, constructorArgs, privateData)`
 

lib/constructs/interface.js

@@ -81,7 +81,7 @@ class LegacyFactoryFunction {
     // This implements the WebIDL legacy factory function behavior, as well as support for overridding
     // the return type, which is used by HTML's element legacy factory functions:
     str += `
-      const thisArgument = exports.new(globalObject);
+      const thisArgument = exports.new(globalObject, new.target);
       const result = Impl.legacyFactoryFunction.call(thisArg, ${formatArgs(setupArgs)});
       return utils.tryWrapperForImpl(utils.isObject(result) ? result : thisArgument);
     `;
@@ -1206,17 +1206,25 @@ class Interface {
 
   generateIface() {
     this.str += `
-      function makeWrapper(globalObject) {
+      function makeWrapper(globalObject, newTarget = undefined) {
         if (globalObject[ctorRegistrySymbol] === undefined) {
           throw new Error('Internal error: invalid global object');
         }
 
-        const ctor = globalObject[ctorRegistrySymbol]["${this.name}"];
-        if (ctor === undefined) {
-          throw new Error('Internal error: constructor ${this.name} is not installed on the passed global object');
+        let prototype;
+        if (newTarget !== undefined) {
+          ({ prototype } = newTarget);
         }
 
-        return Object.create(ctor.prototype);
+        if (!utils.isObject(prototype)) {
+          const ctor = globalObject[ctorRegistrySymbol]["${this.name}"];
+          if (ctor === undefined) {
+            throw new Error('Internal error: constructor ${this.name} is not installed on the passed global object');
+          }
+          ({ prototype } = ctor);
+        }
+
+        return Object.create(prototype);
       }
     `;
 
@@ -1284,8 +1292,8 @@ class Interface {
         return wrapper;
       };
 
-      exports.new = globalObject => {
-        ${this.isLegacyPlatformObj ? "let" : "const"} wrapper = makeWrapper(globalObject);
+      exports.new = (globalObject, newTarget = undefined) => {
+        ${this.isLegacyPlatformObj ? "let" : "const"} wrapper = makeWrapper(globalObject, newTarget);
 
         exports._internalSetup(wrapper, globalObject);
         Object.defineProperty(wrapper, implSymbol, {