Add example for Reflect.construct newTarget parameter (mdn#35996)

files/en-us/web/javascript/reference/global_objects/reflect/construct/index.md

@@ -25,7 +25,7 @@ Reflect.construct(target, argumentsList, newTarget)
 - `argumentsList`
   - : An [array-like object](/en-US/docs/Web/JavaScript/Guide/Indexed_collections#working_with_array-like_objects) specifying the arguments with which `target` should be called.
 - `newTarget` {{optional_inline}}
-  - : The value of [`new.target`](/en-US/docs/Web/JavaScript/Reference/Operators/new.target) operator, which usually specifies the prototype of the returned object. If `newTarget` is not present, its value defaults to `target`.
+  - : The value of the [`new.target`](/en-US/docs/Web/JavaScript/Reference/Operators/new.target) expression inside `target`. Defaults to `target`. Generally ([see example](#changing_new.target)), `target` specifies the _logic_ to initialize the object, while `newTarget.prototype` specifies the _prototype_ of the constructed object.
 
 ### Return value
 
@@ -76,6 +76,74 @@ d instanceof Date; // true
 d.getFullYear(); // 1776
 ```
 
+### Changing new.target
+
+If `newTarget` is passed, it changes the value of `new.target` inside the constructor. The constructed object will be an instance of `newTarget`, not `target`.
+
+```js
+function OneClass() {
+  console.log("OneClass executed");
+  console.log(`new.target is ${new.target.name}`);
+}
+
+function OtherClass() {
+  console.log("OtherClass executed");
+  console.log(`new.target is ${new.target.name}`);
+}
+
+const obj1 = Reflect.construct(OneClass, []);
+// Logs:
+// OneClass executed
+// new.target is OneClass
+console.log(obj1 instanceof OneClass); // true
+
+const obj2 = Reflect.construct(OneClass, [], OtherClass);
+// Logs:
+// OneClass executed
+// new.target is OtherClass
+console.log(obj2 instanceof OtherClass); // true
+console.log(obj2 instanceof OneClass); // false
+```
+
+Of course, there's no strong guarantee about the prototype chain of the constructed object, as it depends on the constructor's implementation. For example, if the `target` constructor returns an object, then that object will be the constructed object, regardless of the `newTarget` value. If `target` is a proxy with a `construct` trap, then the trap fully controls the construction process.
+
+```js
+function OneClass() {
+  return { name: "one" };
+}
+
+function OtherClass() {
+  return { name: "other" };
+}
+
+const obj1 = Reflect.construct(OneClass, [], OtherClass);
+console.log(obj1.name); // 'one'
+console.log(obj1 instanceof OneClass); // false
+console.log(obj1 instanceof OtherClass); // false
+```
+
+A valid `new.target` should be a constructor function with a [`prototype`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/prototype) property, but the latter is not enforced. If the `prototype` property's value is not an object, the initialized object will inherit from `Object.prototype`.
+
+```js
+function OneClass() {
+  console.log("OneClass executed");
+  console.log(`new.target is ${new.target.name}`);
+}
+
+function OtherClass() {
+  console.log("OtherClass executed");
+  console.log(`new.target is ${new.target.name}`);
+}
+
+OtherClass.prototype = null;
+
+const obj = Reflect.construct(OneClass, [], OtherClass);
+// Logs:
+// OneClass executed
+// new.target is OtherClass
+console.log(Object.getPrototypeOf(obj) === Object.prototype); // true
+```
+
 ### Reflect.construct() vs. Object.create()
 
 Prior to the introduction of `Reflect`, objects could be constructed using an arbitrary combination of constructors and prototypes using {{jsxref("Object.create()")}}.