Prototype VS. __proto__ VS. [[Prototype]]

When creating a function, a property object called prototype is being created automatically (you didn‘t create it yourself) and is being attached to the function object (the constructor).
Note: This new prototype object also points to, or has an internal-private link to, the native JavaScript Object.

Example:

function Foo () {
    this.name = ‘John Doe‘;
}

// Foo has an object property called prototype.
// prototype was created automatically when we declared the function Foo.
Foo.hasOwnProperty(‘prototype‘); // true

// Now, we can assign properties to to the prototype object without declaring it first.
Foo.prototype.myName = function () {
    return ‘My name is ‘ + this.name;
}

If you will create a new object out of Foo using the new keyword, you basically creating (among other things) a new object that has an internal or private link to the function‘s prototype Foo we discussed earlier:

var b = new Foo();

b.[[Prototype]] === Foo.prototype  // true

 

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The private linkage to that function‘s object called [[Prototype]]. Many browsers are providing us with a public linkage instead that called __proto__!

 

To be more specific, __proto__ is actually a getter function that belong to the native JavaScript Object and returns the internal-private prototype linkage of whatever the this binding is (returns the [[Prototype]] of b):

b.__proto__ === Foo.prototype // true

It is worth noting that starting of ECMAScript5, you can also use the getPrototypeOf method to get the internal private linkage:

Object.getPrototypeOf(b) === b.__proto__ // true

 

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NOTE: this answer doesn‘t intend to cover the whole process of creating new objects or new constructors, but to help better understand what is __proto__, prototype and [[Prototype]] and how it works.