JavaScript WTF #3: Foo.prototype is not prototype of Foo

Consider the following code:

function Foo() {}
var fooPrototype = Object.getPrototypeOf(Foo);
console.log(Foo.prototype === fooPrototype); // false

Why is it false? Because Foo.prototype is in fact not the prototype of Foo, but the prototype of objects created via expression new Foo():

var obj = new Foo();
var objPrototype = Object.getPrototypeOf(obj);
console.log(Foo.prototype === objPrototype); // true

As for the actual prototype of Foo, it is Function.prototype:

console.log(fooPrototype === Function.prototype); // true

This is very confusing for someone who is trying to learn about prototypes. Although JavaScript is built on prototypes, it is inexplicably shy about discovering and working with prototypes. To the best of my knowledge, Object.getPrototypeOf() was added only in EcmaScript 5.1, 14 years after JavaScript’s inception. For older versions of JavaScript it is recommended to use obj.constructor.prototype to retrieve prototype of obj. This is rather indirect, and it would fail is someone reassign the constructor property.

To visualize how ridiculous this situation is, let’s mentally transfer it to another language. The concept of prototypes is as central to JavaScript as the concept of base (super) classes to object-oriented languages. A similar situation in Java would mean that:

  • There is no keyword super.
  • You can refer to the superclass portion of your object as this.constructor.super.
  • There is a non-standard property __super__ supported only on some JVMs, but not others.
  • Method Object.getSuperOf() is finally added in Java 7.
  • For any class C, expression C.super yields Class.class.
  • This would make Java language a little bit of a mess, and we would ask WTF, wouldn’t we?

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