javascript - 构造函数可以返回什么值来避免返回 this？

Question

当使用 new 调用构造函数时，Javascript 中的 return 语句在什么情况下可以返回 this 以外的值> 关键字？

例子:

function Foo () {
  return something;
}

var foo = new Foo ();

如果我没记错的话，如果something 是一个非函数原语，this 将被返回。否则返回 something。这是正确的吗？

换句话说，something 可以取什么值来导致 (new Foo () instanceof Foo) === false？

Answer 1

在 [[Construct]] 中描述了确切的条件 new 使用的内部属性运算符(operator):

来自 ECMA-262 3rd。版本说明:

13.2.2 [[Construct]]

When the [[Construct]] property for a Function object F is called, the following steps are taken:

1. Create a new native ECMAScript object.
2. Set the [[Class]] property of Result(1) to "Object".
3. Get the value of the prototype property of F.
4. If Result(3) is an object, set the [[Prototype]] property of Result(1) to Result(3).
5. If Result(3) is not an object, set the [[Prototype]] property of Result(1) to the original Object prototype object as described in 15.2.3.1.
6. Invoke the [[Call]] property of F, providing Result(1) as the this value and providing the argument list passed into [[Construct]] as the argument values.
7. If <a href="http://bclary.com/2004/11/07/#a-8" rel="noreferrer noopener nofollow">Type</a>(Result(6)) is Object then return Result(6).
8. Return Result(1).

看第7步和第8步，只有当Result(6) 的类型(从 F 构造函数返回的值函数)不是对象。