Javascript：为什么classList不是Element.prototype的属性

Question

Why is classList not a property of Element.prototype ? Because elements have a classList property, I would automatically assume it to be in the element's prototype , But apparently it's not. So where is it?

Chrome's console:

> Element.prototype.classList
undefined
> document.createElement().classList
DOMTokenList

Answer 1

It has to be a property of each Element instance, because if it was on the Element prototype then all elements would share the same class list, just as an Array 's .length property has to be on each array instance or all arrays would have the same length. The point of the prototype is that it is shared by all objects of the same type.

Answer 2

In addition to nnnnnn 's answer, most instance variables are assigned in the constructor.

function foo(){
    this.bar = "baz";
}

foo.prototype.bar // undefined
new foo().bar // "baz"

Functions are added to the prototype (usually, but they can also be set in the constructor).