为什么构造函数只能返回一个对象？
如果有一个像
function a() {}
then
(new a) instanceof a === true
这样的构造函数但另一方面，
function a() { return {} }
结果是
(new a) instanceof a === false
所以我在想的是，这
function a() { return 123 }
会导致同样的结果。但是，当返回一个 Number 时，
(new a) instanceof a === true
这怎么可能呢？为什么我不能让构造函数返回对象以外的东西？
（我确实知道让构造函数返回数字是相当无用的，但我想了解这种行为的“原因”）
If there is a constructor like
function a() {}
then
(new a) instanceof a === true
But on the other hand,
function a() { return {} }
results in
(new a) instanceof a === false
So what I was thinking is that
function a() { return 123 }
would result in the same thing. However, when returning a Number,
(new a) instanceof a === true
How is this possible? Why can't I make a constructor return something else than an Object?
(I do know making a constructor returning a Number is rather useless but I would like to understand the 'why' of this behaviour)
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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。如果构造函数不返回对象（而是
undefined或其他一些原始值），则结果是新创建的对象。如果允许基元，那么所有构造函数都必须显式返回某些内容（通常为“
this”），否则结果将是未定义（因为函数的结果没有返回是未定义）。那将是一个不必要的麻烦。此外，可以依赖
new来始终返回一个对象，这是有道理的。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 (butundefinedor some other primitive value), the result is the newly created object.If primitives were allowed, then all constructors would have to explicitly return something (typically "
this"), otherwise the result would beundefined(because the result of a function without areturnisundefined). That would be a needless hassle.Additionally, it makes sense that
newcan be relied on to always return an object.