构造函数可以返回哪些值来避免返回 this? [英] What values can a constructor return to avoid returning this?

问题描述

当使用 newreturn 语句可以返回除 this 以外的值的确切情况是什么?> 关键字?

What are the exact circumstances for which a return statement in Javascript can return a value other than this when a constructor is invoked using the new keyword?

示例:

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?

If I'm not mistaken, if something is a non-function primitive, this will be returned. Otherwise something is returned. Is this correct?

换句话说，something 取什么值可以导致 (new Foo () instanceof Foo) === false?

In other words, what values can something take to cause (new Foo () instanceof Foo) === false?

推荐答案

[[Construct]] 内部属性描述了确切的条件，new 运算符:

The exact condition is described on the [[Construct]] internal property, which is used by the new operator:

来自 ECMA-262 3rd.版本规范:

From the ECMA-262 3rd. Edition Specification:

13.2.2 [[构造]]

当 Function 对象 F 的 [[Construct]] 属性是调用，采取以下步骤:

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

1. 创建一个新的原生 ECMAScript 对象.
2. 设置[[Class]]Result(1) 的属性为 "Object".
3. 获取F的prototype属性的值.
4. 如果 Result(3) 是一个对象，则设置 [[Prototype]] Result(1) 到 Result(3) 的属性.
5. 如果 Result(3) 不是对象，则设置 [[Prototype]] 将Result(1)的属性转换为原始的Object原型对象作为15.2.3.1 中有所描述.
6. 调用[[Call]]F 的 属性，提供 Result(1) 作为 this 值和提供传入 [[Construct]] 的参数列表作为参数值.
7. 如果 输入(Result(6))是Object 然后返回 Result(6).
8. 返回Result(1).

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 Type(Result(6)) is Object then return Result(6).
8. Return Result(1).

看第 7 步和第 8 步，新对象只有在Result(6) 的类型(从 F 构造函数返回的值函数)不是一个对象.

Look at steps 7 and 8, the new object will be returned only if the type of Result(6) (the value returned from the F constructor function) is not an Object.

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