[javascript] What is the 'new' keyword in JavaScript?

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?

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.

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.

 " Every object (including functions) has this internal property called [[prototype]]" 

Every function has a proto- type object that’s automatically set as the prototype of the objects created with that function.

you guys can check easily:

const a = { name: "something" };
console.log(a.prototype); // undefined because it is not directly accessible

const b = function () {
  console.log("somethign");};

console.log(b.prototype); // returns b {}

But every function and objects has __proto__ property which points to the prototype of that object or function. __proto__ and prototype are 2 different terms. I think we can make this comment: "Every object is linked to a prototype via the proto " But __proto__ does not exist in javascript. this property is added by browser just to help for debugging.

console.log(a.__proto__); // returns {}
console.log(b.__proto__); // returns [Function]

You guys can check this on the terminal easily. So what is constructor function.

function CreateObject(name,age){
    this.name=name;
    this.age =age
}

5 things that pay attention first:

1- When constructor function is invoked with new, the function’s internal [[Construct]] method is called to create a new instance object and allocate memory.

2- We are not using return keyword. new will handle it.

3- Name of the function is capitalized so when developers see your code they can understand that they have to use new keyword.

4- We do not use arrow function. Because the value of the this parameter is picked up at the moment that the arrow function is created which is "window". arrow functions are lexically scoped, not dynamically. Lexically here means locally. arrow function carries its local "this" value.

5- Unlike regular functions, arrow functions can never be called with the new keyword because they do not have the [[Construct]] method. The prototype property also does not exist for arrow functions.

const me=new CreateObject("yilmaz","21")

new invokes the function and then creates an empty object {} and then adds "name" key with the value of "name", and "age" key with the value of argument "age".

When we invoke a function, a new execution context is created with "this" and "arguments", that is why "new" has access to these arguments.

By default this inside the constructor function will point to the "window" object, but new changes it. "this" points to the empty object {} that is created and then properties are added to newly created object. If you had any variable that defined without "this" property will no be added to the object.

function CreateObject(name,age){
    this.name=name;
    this.age =age;
    const myJob="developer"
}

myJob property will not added to the object because there is nothing referencing to the newly created object.

   const me= {name:"yilmaz",age:21} // there is no myJob key

in the beginning I said every function has "prototype" property including constructor functions. We can add methods to the prototype of the constructor, so every object that created from that function will have access to it.

 CreateObject.prototype.myActions=function(){ //define something}

Now "me" object can use "myActions" method.

javascript has built-in constructor functions: Function,Boolean,Number,String..

if I create

const a = new Number(5);
console.log(a);  // [Number: 5]
console.log(typeof a); // object

Anything that created by using new has type of object. now "a" has access all of the methods that are stored inside Number.prototype. If I defined

const b = 5;
console.log(a === b);//false

a and b are 5 but a is object and b is primitive. even though b is primitive type, when it is created, javascript automatically wraps it with Number(), so b has access to all of the methods that inside Number.prototype.

Constructor function is useful when you want to create multiple similar objects with the same properties and methods. That way you will not be allocating extra memory so your code will run more efficiently.

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'

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 :)

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(Object.getPrototypeOf(doggie) === 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.

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.

Javascript is not object oriented programming(OOP) language therefore the LOOK UP process in javascript work using 'DELEGATION PROCESS' also known as prototype delegation or prototypical inheritance.

If you try to get the value of a property from an object that it doesn't have, the JavaScript engine looks to the object's prototype (and its prototype, 1 step above at a time) it's prototype chain untll the chain ends upto null which is Object.prototype == null (Standard Object Prototype). At this point if property or method is not defined than undefined is returned.

Thus with the new keyword some of the task that were manually done e.g

1. Manual Object Creation e.g newObj.
2. Hidden bond Creation using proto (aka: dunder proto) in JS spec [[prototype]] (i.e. proto)
3. referencing and assign properties to newObj
4. return of newObj object.

All is done manually.

function CreateObj(value1, value2) {
  const newObj = {};
  newObj.property1 = value1;
  newObj.property2 = value2;
  return newObj;
}
var obj = CreateObj(10,20);

obj.__proto__ === Object.prototype;              // true
Object.getPrototypeOf(obj) === Object.prototype // true

Javascript Keyword new helps to automate this process:

1. new object literal is created identified by this:{}
2. referencing and assign properties to this
3. Hidden bond Creation [[prototype]] (i.e. proto) to Function.prototype shared space.
4. implicit return of this object {}

function CreateObj(value1, value2) {
  this.property1 = value1;
  this.property2 = value2;
}

var obj = new CreateObj(10,20);
obj.__proto__ === CreateObj.prototype             // true
Object.getPrototypeOf(obj) == CreateObj.prototype // true

Calling Constructor Function without the new Keyword:

=> this: Window

function CreateObj(value1, value2) {
  var isWindowObj = this === window;
  console.log("Is Pointing to Window Object", isWindowObj);
  this.property1 = value1;
  this.property2 = value2;
}
var obj = new CreateObj(10,20); // Is Pointing to Window Object false
var obj = CreateObj(10,20); // Is Pointing to Window Object true
window.property1; // 10
window.property2; // 20

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.

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.

Quoting David Flanagan from JavaScripit: The Definitive Guide (6th Edition),Ch. 4, Page # 62:

When an object creation expression is evaluated, JavaScript first creates a new empty object, just like the one created by the object initializer {}. Next, it invokes the specified function with the specified arguments, passing the new object as the value of the this keyword. The function can then use this to initialize the properties of the newly created object. Functions written for use as constructors do not return a value, and the value of the object creation expression is the newly created and initialized object. If a constructor does return an object value, that value becomes the value of the object creation expression and the newly created object is discarded.

---Additional Info---

The functions used in code snippet of above cases have special names in JS world as below:

Case I and II - Constructor function

Case III - Factory function. Factory functions shouldn't be used with new keyword which I've done to explain the concept in current thread.

You can read about difference between them in this thread.

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.

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();

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.

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.