What is the 'new' keyword in JavaScript?

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The new keyword in JavaScript can be quite confusing when it is first encountered, as people tend to think that JavaScript is not an object-oriented programming language.

  • What is it?
  • What problems does it solve?
  • When is it appropriate and when not?

It does 5 things:

  1. It creates a new object. The type of this object is simply object.
  2. It sets this new object's internal, inaccessible, [[prototype]] (i.e. __proto__) property to be the constructor function's external, accessible, prototype object (every function object automatically has a prototype property).
  3. It makes the this variable point to the newly created object.
  4. It executes the constructor function, using the newly created object whenever this is mentioned.
  5. It returns the newly created object, unless the constructor function returns a non-null object reference. In this case, that object reference is returned instead.

Note: constructor function refers to the function after the new keyword, as in

new ConstructorFunction(arg1, arg2)

Once this is done, if an undefined property of the new object is requested, the script will check the object's [[prototype]] object for the property instead. This is how you can get something similar to traditional class inheritance in JavaScript.

The most difficult part about this is point number 2. Every object (including functions) has this internal property called [[prototype]]. It can only be set at object creation time, either with new, with Object.create, or based on the literal (functions default to Function.prototype, numbers to Number.prototype, etc.). It can only be read with Object.getPrototypeOf(someObject). There is no other way to set or read this value.

Functions, in addition to the hidden [[prototype]] property, also have a property called prototype, and it is this that you can access, and modify, to provide inherited properties and methods for the objects you make.


Here is an example:

ObjMaker = function() {this.a = 'first';};
// ObjMaker is just a function, there's nothing special about it that makes 
// it a constructor.

ObjMaker.prototype.b = 'second';
// like all functions, ObjMaker has an accessible prototype property that 
// we can alter. I just added a property called 'b' to it. Like 
// all objects, ObjMaker also has an inaccessible [[prototype]] property
// that we can't do anything with

obj1 = new ObjMaker();
// 3 things just happened.
// A new, empty object was created called obj1.  At first obj1 was the same
// as {}. The [[prototype]] property of obj1 was then set to the current
// object value of the ObjMaker.prototype (if ObjMaker.prototype is later
// assigned a new object value, obj1's [[prototype]] will not change, but you
// can alter the properties of ObjMaker.prototype to add to both the
// prototype and [[prototype]]). The ObjMaker function was executed, with
// obj1 in place of this... so obj1.a was set to 'first'.

obj1.a;
// returns 'first'
obj1.b;
// obj1 doesn't have a property called 'b', so JavaScript checks 
// its [[prototype]]. Its [[prototype]] is the same as ObjMaker.prototype
// ObjMaker.prototype has a property called 'b' with value 'second'
// returns 'second'

It's like class inheritance because now, any objects you make using new ObjMaker() will also appear to have inherited the 'b' property.

If you want something like a subclass, then you do this:

SubObjMaker = function () {};
SubObjMaker.prototype = new ObjMaker(); // note: this pattern is deprecated!
// Because we used 'new', the [[prototype]] property of SubObjMaker.prototype
// is now set to the object value of ObjMaker.prototype.
// The modern way to do this is with Object.create(), which was added in ECMAScript 5:
// SubObjMaker.prototype = Object.create(ObjMaker.prototype);

SubObjMaker.prototype.c = 'third';  
obj2 = new SubObjMaker();
// [[prototype]] property of obj2 is now set to SubObjMaker.prototype
// Remember that the [[prototype]] property of SubObjMaker.prototype
// is ObjMaker.prototype. So now obj2 has a prototype chain!
// obj2 ---> SubObjMaker.prototype ---> ObjMaker.prototype

obj2.c;
// returns 'third', from SubObjMaker.prototype

obj2.b;
// returns 'second', from ObjMaker.prototype

obj2.a;
// returns 'first', from SubObjMaker.prototype, because SubObjMaker.prototype 
// was created with the ObjMaker function, which assigned a for us

I read a ton of rubbish on this subject before finally finding this page, where this is explained very well with nice diagrams.

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Suppose you have this function:

var Foo = function(){
  this.A = 1;
  this.B = 2;
};

If you call this as a standalone function like so:

Foo();

Executing this function will add two properties to the window object (A and B). It adds it to the window because window is the object that called the function when you execute it like that, and this in a function is the object that called the function. In Javascript at least.

Now, call it like this with new:

var bar = new Foo();

What happens when you add new to a function call is that a new object is created (just var bar = new Object()) and that the this within the function points to the new Object you just created, instead of to the object that called the function. So bar is now an object with the properties A and B. Any function can be a constructor, it just doesn't always make sense.

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In addition to Daniel Howard's answer, here is what new does (or at least seems to do):

function New(func) {
    var res = {};
    if (func.prototype !== null) {
        res.__proto__ = func.prototype;
    }
    var ret = func.apply(res, Array.prototype.slice.call(arguments, 1));
    if ((typeof ret === "object" || typeof ret === "function") && ret !== null) {
        return ret;
    }
    return res;
}

While

var obj = New(A, 1, 2);

is equivalent to

var obj = new A(1, 2);

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For beginners to understand it better

try out the following code in the browser console.

function Foo() { 
    return this; 
}

var a = Foo();       //returns window object
var b = new Foo();   //returns empty object of foo

a instanceof Window;  // true
a instanceof Foo;     // false

b instanceof Window;  // false
b instanceof Foo;     // true

Now you can read the community wiki answer :)

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so it's probably not for creating instances of object

It's used exactly for that. You define a function constructor like so:

function Person(name) {
    this.name = name;
}

var john = new Person('John');

However the extra benefit that ECMAScript has is you can extend with the .prototype property, so we can do something like...

Person.prototype.getName = function() { return this.name; }

All objects created from this constructor will now have a getName because of the prototype chain that they have access to.

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Comments
  • Also, related thread - stackoverflow.com/questions/383402/…
  • read these examples first folks, developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…
  • Just wanted to add: There is in fact a way to access the internal [[prototype]], by __proto__. This is however non-standard, and only supported by relatively new browsers (and not all of them). There is a standardized way coming up, namely Object.getPrototypeOf(obj), but it is Ecmascript3.1, and is itself only supported on new browers - again. It is generally recommended to not use that property though, stuff gets complicated real fast inside there.
  • Question: what happens differently if ObjMaker is defined as a function that returns a value?
  • @LonelyPixel new exists so that you don't have to write factory methods to construct/copy functions/objects. It means, "Copy this, making it just like its parent 'class'; do so efficiently and correctly; and store inheritance info that is accessible only to me, JS, internally". To do so, it modifies the otherwise inaccessible internal prototype of the new object to opaquely encapsulate the inherited members, mimicking classical OO inheritance chains (which aren't runtime modifiable). You can simulate this without new, but inheritance will be runtime modifiable. Good? Bad? Up to you.
  • a small point to add: a call to a constructor, when preceded by the new keyword, automatically returns the created object; there is no need to explicitly return it from within the constructor.
  • There is a note that says Notice that this pattern is deprecated!. What is the correct up-to-date pattern to set the prototype of a class?
  • Depends on execution context. In my case (Qt scripting) it's just a global object.
  • will this cause more memory usage?
  • because window is the object that called the function - must be: because window is the object that contains the function.
  • @Taurus In a web browser a non-method function will be a method of window implicitly. Even in a closure, even if anonymus. However, in the example it is a simple method invocation on window: Foo(); => [default context].Foo(); => window.Foo();. In this expression window is the context (not only the caller, which does not matter).
  • @Taurus Basicly yes. However in ECMA 6 and 7 things are more complex (see lambdas, classes, etc).
  • I found that javascript is easier to understand than english :v