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开发者_JAVA技巧 is it?
• What problems does it solve?
• When is it appropriate and when not?

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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.

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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

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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 stand-alone 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();

When you add new to a function call, a new object is created (just var bar = new Object()) and 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 the 'window' object
var b = new Foo();   // Returns an 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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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.

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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 assignment operator = says: We are going to initialize this variable with the expression after the =
3. The expression is new Dog(12). The JavaScript 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.

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Please take a look at my observation on case III below. It is about what happens when you have an explicit return statement in a function which you are newing up. Have a look at the 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 variable Foo. When you call this function it returns undefined. Since there isn’t any explicit return statement, the JavaScript interpreter forcefully inserts a return undefined; statement at the end of the function. So the above code sample is equivalent to:

var Foo = function(){
  this.A = 1;
  this.B = 2;
  return undefined;
};
console.log(Foo()); //prints undefined
console.log(window.A); //prints 1

When Foo function is invoked window is the default 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 the 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, 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, the 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 the new keyword.

Quoting David Flanagan from JavaScript: The Definitive Guide (6th Edition), Chapter 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 information:

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

Case #	Name
Case I	Constructor function
Case II	Constructor function
Case III	Factory function

You can read about the difference between constructor functions and factory functions in this thread.

Code smell in case III - Factory functions should not be used with the new keyword which I've shown in the code snippet above. I've done so deliberately only to explain the concept.

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

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

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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.

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

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Every function has a prototype 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 the __proto__ property which points to the prototype of that object or function. __proto__ and prototype are two 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 a constructor function?

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

Five things that pay attention first:

1. When the 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. The name of the function is capitalized, so when developers see your code they can understand that they have to use the new keyword.

4. We do not use the 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. The 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", and 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 isn't any 'myJob' key

In the beginning I said every function has a "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 the "myActions" method.

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

If I create

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

Anything that is created by using new has the 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.

A 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.

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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 a capital letter for objects that are supposed to be instantiated by the new keyword.

obj = new Element();

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JavaScript is not an object-oriented programming (OOP) language. Therefore the look up process in JavaScript works using a 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, one step above at a time). It's prototype chain until the chain ends up to null which is Object.prototype == null (Standard Object Prototype).

At this point, if the property or method is not defined then undefined is returned.

Important! Functions are are first-class objects.

Functions = Function + Objects Combo

FunctionName.prototype = { shared SubObject }

{
  // other properties
  prototype: {
   // shared space which automatically gets [[prototype]] linkage
      when "new" keyword is used on creating instance of "Constructor
      Function"
  }
}

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 the JavaScript specification [[prototype]] (i.e., proto)
3. referencing and assign properties to newObj
4. return of the 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. A 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 a 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

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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'

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Well, JavaScript per se can differ greatly from platform to platform as it is always an implementation of the original specification ECMAScript (ES).

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 a 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 specifications:

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.