How exactly does Javascript instanceof work? Is it slow style?

Question

How does the performance of instanceof fair for \"huge libraries\"?

Does it travel up the prototype chain one by one, similar to this?

Answer 1

in V8 (Chrome's JS engine), there seems to be little-to-no performance hit:

> function A(){}
> function B(){}
> function C(){}
> function D(){}
> B.prototype = new A();
> C.prototype = new B();
> D.prototype = new C();
> 
> var objA = new A();
> var objD = new D();
> 
> var start = (+new Date()); for(var i=0; i<10000000; i++){ objA instanceof A } console.log((+new Date()) - start);
138
> var start = (+new Date()); for(var i=0; i<10000000; i++){ objD instanceof A } console.log((+new Date()) - start);
138

Firefox shows identical behavior.

Going a bit crazy here, but:

> var classes = [];
> for(var i=0; i<10000; i++){
>   classes[i] = function(){};
>   i && (classes[i].prototype = new (classes[i-1])());
> }
>
> var obj0 = new classes[0],
>  obj9999 = new classes[9999];
>
> var start = (+new Date()); for(var i=0; i<10000000; i++){ obj0   instanceof classes[0] } console.log((+new Date()) - start);
138
> var start = (+new Date()); for(var i=0; i<10000000; i++){ obj999 instanceof classes[0] } console.log((+new Date()) - start);
138

I think it's safe to assume there is no performance hit if it can drill through 10,000 classes and not see 1 ms performance difference :)