__proto__ VS. prototype in JavaScript

Question

This figure again shows that every object has a prototype. Constructor function Foo also has its own __proto__ which is Function.prototype, a

Answer 1

prototype is a property of a Function object. It is the prototype of objects constructed by that function.

__proto__ is internal property of an object, pointing to its prototype. Current standards provide an equivalent Object.getPrototypeOf(O) method, though de facto standard __proto__ is quicker.

You can find instanceof relationships by comparing a function's prototype to an object's __proto__ chain, and you can break these relationships by changing prototype.

function Point(x, y) {
    this.x = x;
    this.y = y;
}

var myPoint = new Point();

// the following are all true
myPoint.__proto__ == Point.prototype
myPoint.__proto__.__proto__ == Object.prototype
myPoint instanceof Point;
myPoint instanceof Object;

Here Point is a constructor function, it builds an object (data structure) procedurally. myPoint is an object constructed by Point() so Point.prototype gets saved to myPoint.__proto__ at that time.