为什么构造函数只能返回一个对象? [英] Why can a constructor only return an object?

问题描述

如果有类似的构造函数

function a() {}

然后

(new a) instanceof a === true

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但另一方面，

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But on the other hand,

function a() { return {} }

结果

(new a) instanceof a === false

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所以我的想法是

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So what I was thinking is that

function a() { return 123 }

会导致同样的事情.然而，当返回一个数字时，

would result in the same thing. However, when returning a Number,

(new a) instanceof a === true

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这怎么可能?为什么我不能让构造函数返回对象以外的东西?

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How is this possible? Why can't I make a constructor return something else than an Object?

(我知道让构造函数返回一个数字是相当无用的，但我想了解这种行为的原因")

(I do know making a constructor returning a Number is rather useless but I would like to understand the 'why' of this behaviour)

推荐答案

根据规范:如果调用构造函数返回一个对象，则该对象是 new 表达式的结果.如果构造函数没有返回对象(但是undefined 或其他一些原始值)，则结果是新创建的对象.

According to the spec: If calling the constructor returns an object, then this object is the result of the new-expression. If the constructor doesn't return an object (but undefined or some other primitive value), the result is the newly created object.

如果允许基元，那么所有构造函数都必须显式返回一些东西(通常是this")，否则结果将是undefined(因为没有 return 的函数是 undefined).那将是一个不必要的麻烦.

If primitives were allowed, then all constructors would have to explicitly return something (typically "this"), otherwise the result would be undefined (because the result of a function without a return is undefined). That would be a needless hassle.

此外，可以依赖 new 来始终返回一个对象.

Additionally, it makes sense that new can be relied on to always return an object.

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