javascript - Why can a constructor only return an object? -

if there constructor like

function a() {} 

then

(new a) instanceof === true 

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but on other hand,

function a() { return {} } 

results in

(new a) instanceof === false 

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so thinking that

function a() { return 123 } 

would result in same thing. however, when returning number,

(new a) instanceof === true 

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how possible? why can't make constructor return else object?

(i know making constructor returning number rather useless understand 'why' of behaviour)

according spec: if calling constructor returns object, object result of new-expression. if constructor doesn't return object (but undefined or other primitive value), result newly created object.

if primitives allowed, constructors have explicitly return (typically "this"), otherwise result undefined (because result of function without return undefined). needless hassle.

additionally, makes sense new can relied on return object.