Why can a constructor only return an object?

If there is a constructor like

function a() {}

then

(new a) instanceof a === true

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But on the other hand,

function a() { return {} }

results in

(new a) instanceof a === false

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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开发者_StackOverflow社区

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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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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.

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.

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