将新数组分配给NodeList.prototype

Question

Why is not possible to inherit array props and methods when I set NodeList.prototype to Array object like this - NodeList.prototype = new Array; and I can do that with my custom constructor prototype ? Why first a object does not inherit array methods and aa does ? Any explanation or blog post link about this is welcome.

function A(a) {
 this.a = a;
}

A.prototype.b = 'b';

var a = new A('a');

A.prototype = new Array; // aa instance will now have map, filter, some ...

var aa = new A('aa');

Answer 1

It doesn't work because the prototype chain is set at construction time. Calling A.prototype = new Array; will only affect instances created after that.

To change the prototype of an object after it has been instantiated, you could use Object.setPrototypeOf(a)

Lastly, note that in your example, you are obliterating the existing prototype so all new instances of A will not have the original b: "b" property.

Answer 2

Changing the prototype of native objects (eg NodeList) is always risky. It might or might not work. It's much better to extend the prototype or extend the actual object.

NodeList.prototype.map = Array.prototype.map;
// or:
someNodeList.map = Array.prototype.map;

And to exaplain you example: When you create a new object, it takes the prototype of the constructor and stores its reference to its __proto__ property. So if you completely replace the prototype of some constructor, it will affect only newly created objects, because they will have a reference to the new prototype.