What is the difference between `.prototype` and `.__proto__`?

Question

This figure again shows that every object has a prototype. Constructor function Foo also has its own __proto__ which is Function.prototype, and which in turn also references via its __proto__ property again to the Object.prototype. Thus, repeat, Foo.prototype is just an explicit property of Foo which refers to the prototype of b and c objects.

var b = new Foo(20);
var c = new Foo(30);

What are the differences between __proto__ and prototype?

The figure was taken from dmitrysoshnikov.com.

Note: there is now a 2nd edition (2017) to the above 2010 article.

Answer 1

__proto__ is the actual object that is used in the lookup chain to resolve methods, etc. prototype is the object that is used to build __proto__ when you create an object with new:

( new Foo ).__proto__ === Foo.prototype
( new Foo ).prototype === undefined