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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.

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);

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 :)

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.

JavaScript is an object-oriented programming language and it's used exactly for creating instances. It's prototype-based, rather than class-based, but that does not mean that it is not object-oriented.

Javascript is a dynamic programming language which supports the object oriented programming paradigm, and it use used for creating new instances of object.

Classes are not necessary for objects - Javascript is a prototype based language.

sometimes code is easier than words:

var func1 = function (x) { this.x = x; }                    // used with 'new' only

var func2 = function (x) { var z={}; z.x = x; return z; }   // used both ways

func1.prototype.y = 11;

func2.prototype.y = 12;



A1 = new func1(1);      // has A1.x  AND  A1.y

A2 =     func1(1);      // undefined ('this' refers to 'window')

B1 = new func2(2);      // has B1.x  ONLY

B2 =     func2(2);      // has B2.x  ONLY

for me, as long as I not prototype, I use style of func2 as it gives me a bit more flexibility inside and outside the function.

There are already some very great answers but I'm posting a new one to emphasize my observation on case III below about what happens when you have an explicit return statement in a function which you are newing up. Have a look at below cases:

Case I:

var Foo = function(){

  this.A = 1; 

  this.B = 2;

};

console.log(Foo()); //prints undefined

console.log(window.A); //prints 1

Above is a plain case of calling the anonymous function pointed by Foo. When you call this function it returns undefined. Since there is no explicit return statement so JavaScript interpreter forcefully inserts a return undefined; statement in the end of the function. Here window is the invocation object (contextual this) which gets new A and B properties.

Case II:

var Foo = function(){

  this.A = 1;

  this.B = 2;

};

var bar = new Foo();

console.log(bar()); //illegal isn't pointing to a function but an object

console.log(bar.A); //prints 1

Here JavaScript interpreter seeing the new keyword creates a new object which acts as the invocation object (contextual this) of anonymous function pointed by Foo. In this case A and B become properties on the newly created object (in place of window object). Since you don't have any explicit return statement so JavaScript interpreter forcefully inserts a return statement to return the new object created due to usage of new keyword.

Case III:

var Foo = function(){

  this.A = 1;

  this.B = 2;

  return {C:20,D:30}; 

};

var bar = new Foo();

console.log(bar.C);//prints 20

console.log(bar.A); //prints undefined. bar is not pointing to the object which got created due to new keyword.

Here again JavaScript interpreter seeing the new keyword creates a new object which acts as the invocation object (contextual this) of anonymous function pointed by Foo. Again, A and B become properties on the newly created object. But this time you have an explicit return statement so JavaScript interpreter will not do anything of its own.

The thing to note in case III is that the object being created due to new keyword got lost from your radar. bar is actually pointing to a completely different object which is not the one which JavaScript interpreter created due to new keyword.

The new keyword is for creating new object instances. And yes, javascript is a dynamic programming language, which supports the object oriented programming paradigm. The convention about the object naming is, always use capital letter for objects that are supposed to be instantiated by the new keyword.

obj = new Element();

Well JavaScript per si can differ greatly from platform to platform as it is always an implementation of the original specification EcmaScript.

In any case, independently of the implementation all JavaScript implementations that follow the EcmaScript specification right, will give you an Object Oriented Language. According to the ES standard:

ECMAScript is an object-oriented programming language for
performing computations and manipulating computational objects
within a host environment.

So now that we have agreed that JavaScript is an implementation of EcmaScript and therefore it is an object-oriented language. The definition of the new operation in any Object-oriented language, says that such keyword is used to create an object instance from a class of a certain type (including anonymous types, in cases like C#).

In EcmaScript we don't use classes, as you can read from the specs:

ECMAScript does not use classes such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways including via
a literal notation or via constructors which create objects and then execute code that initializes all or part of them by assigning initial
values to their properties. Each constructor is a function that has a
property named ―
prototype ‖ that is used to implement prototype - based inheritance and shared properties. Objects are created by

using constructors in new expressions; for example, new
Date(2009,11) creates a new Date object. Invoking a constructor
without using new has consequences that depend on the constructor.
For example, Date() produces a string representation of the
current date and time rather than an object.

The new keyword changes the context under which the function is being run and returns a pointer to that context.

When you don't use the new keyword, the context under which function Vehicle() runs is the same context from which you are calling the Vehicle function. The this keyword will refer to the same context. When you use new Vehicle(), a new context is created so the keyword this inside the function refers to the new context. What you get in return is the newly created context.

The new keyword creates instances of objects using functions as a constructor. For instance:

var Foo = function() {};

Foo.prototype.bar = 'bar';



var foo = new Foo();

foo instanceof Foo; // true

Instances inherit from the prototype of the constructor function. So given the example above...

foo.bar; // 'bar'

Summary:

The new keyword is used in javascript to create a object from a constructor function. The new keyword has to be placed before the constructor function call and will do the following things:

1. Creates a new object


2. Sets the prototype of this object to the constructor function's prototype property


3. Binds the this keyword to the newly created object and executes the constructor function


4. Returns the newly created object

Example:

function Dog (age) {

  this.age = age;

}



const doggie = new Dog(12);



console.log(doggie);

console.log(doggie.__proto__ === Dog.prototype) // true

What exactly happens:

1. 
   const doggie says: We need memory for declaring a variable.


2. The assigment operator = says: We are going to initialize this variable with the expression after the =
   


3. The expression is new Dog(12). The JS engine sees the new keyword, creates a new object and sets the prototype to Dog.prototype


4. The constructor function is executed with the this value set to the new object. In this step is where the age is assigned to the new created doggie object.


5. The newly created object is returned and assigned to the variable doggie.