What's the difference between "prototype" and "__proto__"?

yumeng liu

What's the difference between "prototype" and "__proto__" in JavaScript?

Lost User

Sherin_Mathew

__proto__ - __proto__ is the actual object that is used in the lookup chain to resolve methods. - It is a property that all objects have. This is the property which is used by the JavaScript engine for inheritance. - According to ECMA specifications it is supposed to be an internal property, however most vendors allow it to be accessed and modified. Syntax

var Circle = function () {};
var shape = {};
var circle = new Circle();

// Set the object prototype.
// DEPRECATED.This is for example purposes only. DO NOT DO THIS in real code.
shape.__proto__ = circle;

// Get the object prototype
console.log(shape.__proto__ === circle); // true

prototype - prototype is a property belonging only to functions. - It is used to build __proto__ when the function happens to be used as a constructor with the new keyword. - In prototype-based object oriented languages like Self and Javascript, every object in the system has a field that says "if I don't have a property or method that is requested of me, go to the object that this field references my prototype and look for it". - Since that object will also have this "prototype" field as well, this becomes a recursive process. - It is what is meant by a prototype chain. - Note that this means that in a prototype language, there is no abstract concept of a "class" Syntax

var shape = function () {};
var p = {
a: function () {
console.log('aaa');
}
};
shape.prototype.__proto__ = p;

var circle = new shape();
circle.a(); // aaa
console.log(shape.prototype === circle.__proto__); // true

// or
var shape = function () {};
var p = {
a: function () {
console.log('a');
}
};

var circle = new shape();
circle.__proto__ = p;
circle.a(); // a
console.log(shape.prototype === circle.__proto__); // false

// or
function test() {};
test.prototype.myname = function () {
console.log('myname');
};

var a = new test();
console.log(a.__proto__ === test.prototype); // true
a.myname(); // myname

// or
var fn = function () {};
fn.prototype.myname = function () {
console.log('myname');
};

var obj = {
__proto__: fn.prototype
};

obj.myname(); // myname

Lost User

Very interesting, but that question was posted nearly eight years ago. It is most unlikely that the poster is still working on the same issues. Focusing on current questions is a better idea.