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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:
It creates a new object. The type of this object is simply object.
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).
It makes the this variable point to the newly created object.
It executes the constructor function, using the newly created object whenever this is mentioned.
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 get or set 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 just `{}`. 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.
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.
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 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 :)
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.
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:
Creates a new object
Sets the prototype of this object to the constructor function's prototype property
Binds the this keyword to the newly created object and executes the constructor function
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:
const doggie says: We need memory for declaring a variable.
The assignment operator = says: We are going to initialize this variable with the expression after the =
The expression is new Dog(12). The JavaScript engine sees the new keyword, creates a new object and sets the prototype to Dog.prototype
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.
The newly created object is returned and assigned to the variable doggie.
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.
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.
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.
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.
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:
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.
We are not using return keyword. new will handle it.
The name of the function is capitalized, so when developers see your code they can understand that they have to use the new keyword.
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.
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.
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();
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.,
Manual object creation, e.g., newObj.
Hidden bond creation using proto (AKA: dunder proto) in the JavaScript specification [[prototype]] (i.e., proto)
referencing and assign properties to newObj
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:
A new object literal is created identified by this:{}
referencing and assign properties to this
Hidden bond creation [[prototype]] (i.e. proto) to Function.prototype shared space.
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
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'
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.
It has 3 stages:
1.Create: It creates a new object, and sets this object's [[prototype]] property to be the prototype property of the constructor function.
2.Execute: It makes this point to the newly created object and executes the constructor function.
3.Return: In normal case, it will return the newly created object. However, if you explicitly return a non-null object or a function , this value is returned instead. To be mentioned, if you return a non-null value, but it is not an object(such as Symbol value, undefined, NaN), this value is ignored and the newly created object is returned.
function myNew(constructor, ...args) {
const obj = {}
Object.setPrototypeOf(obj, constructor.prototype)
const returnedVal = constructor.apply(obj, args)
if (
typeof returnedVal === 'function'
|| (typeof returnedVal === 'object' && returnedVal !== null)) {
return returnedVal
}
return obj
}
For more info and the tests for myNew, you can read my blog: https://medium.com/#magenta2127/how-does-the-new-operator-work-f7eaac692026
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:
It creates a new object. The type of this object is simply object.
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).
It makes the this variable point to the newly created object.
It executes the constructor function, using the newly created object whenever this is mentioned.
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 get or set 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 just `{}`. 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.
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.
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 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 :)
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.
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:
Creates a new object
Sets the prototype of this object to the constructor function's prototype property
Binds the this keyword to the newly created object and executes the constructor function
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:
const doggie says: We need memory for declaring a variable.
The assignment operator = says: We are going to initialize this variable with the expression after the =
The expression is new Dog(12). The JavaScript engine sees the new keyword, creates a new object and sets the prototype to Dog.prototype
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.
The newly created object is returned and assigned to the variable doggie.
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.
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.
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.
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.
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:
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.
We are not using return keyword. new will handle it.
The name of the function is capitalized, so when developers see your code they can understand that they have to use the new keyword.
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.
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.
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();
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.,
Manual object creation, e.g., newObj.
Hidden bond creation using proto (AKA: dunder proto) in the JavaScript specification [[prototype]] (i.e., proto)
referencing and assign properties to newObj
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:
A new object literal is created identified by this:{}
referencing and assign properties to this
Hidden bond creation [[prototype]] (i.e. proto) to Function.prototype shared space.
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
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'
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.
It has 3 stages:
1.Create: It creates a new object, and sets this object's [[prototype]] property to be the prototype property of the constructor function.
2.Execute: It makes this point to the newly created object and executes the constructor function.
3.Return: In normal case, it will return the newly created object. However, if you explicitly return a non-null object or a function , this value is returned instead. To be mentioned, if you return a non-null value, but it is not an object(such as Symbol value, undefined, NaN), this value is ignored and the newly created object is returned.
function myNew(constructor, ...args) {
const obj = {}
Object.setPrototypeOf(obj, constructor.prototype)
const returnedVal = constructor.apply(obj, args)
if (
typeof returnedVal === 'function'
|| (typeof returnedVal === 'object' && returnedVal !== null)) {
return returnedVal
}
return obj
}
For more info and the tests for myNew, you can read my blog: https://medium.com/#magenta2127/how-does-the-new-operator-work-f7eaac692026
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:
It creates a new object. The type of this object is simply object.
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).
It makes the this variable point to the newly created object.
It executes the constructor function, using the newly created object whenever this is mentioned.
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 get or set 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 just `{}`. 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.
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.
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 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 :)
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.
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:
Creates a new object
Sets the prototype of this object to the constructor function's prototype property
Binds the this keyword to the newly created object and executes the constructor function
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:
const doggie says: We need memory for declaring a variable.
The assignment operator = says: We are going to initialize this variable with the expression after the =
The expression is new Dog(12). The JavaScript engine sees the new keyword, creates a new object and sets the prototype to Dog.prototype
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.
The newly created object is returned and assigned to the variable doggie.
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.
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.
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.
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.
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:
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.
We are not using return keyword. new will handle it.
The name of the function is capitalized, so when developers see your code they can understand that they have to use the new keyword.
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.
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.
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();
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.,
Manual object creation, e.g., newObj.
Hidden bond creation using proto (AKA: dunder proto) in the JavaScript specification [[prototype]] (i.e., proto)
referencing and assign properties to newObj
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:
A new object literal is created identified by this:{}
referencing and assign properties to this
Hidden bond creation [[prototype]] (i.e. proto) to Function.prototype shared space.
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
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'
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.
It has 3 stages:
1.Create: It creates a new object, and sets this object's [[prototype]] property to be the prototype property of the constructor function.
2.Execute: It makes this point to the newly created object and executes the constructor function.
3.Return: In normal case, it will return the newly created object. However, if you explicitly return a non-null object or a function , this value is returned instead. To be mentioned, if you return a non-null value, but it is not an object(such as Symbol value, undefined, NaN), this value is ignored and the newly created object is returned.
function myNew(constructor, ...args) {
const obj = {}
Object.setPrototypeOf(obj, constructor.prototype)
const returnedVal = constructor.apply(obj, args)
if (
typeof returnedVal === 'function'
|| (typeof returnedVal === 'object' && returnedVal !== null)) {
return returnedVal
}
return obj
}
For more info and the tests for myNew, you can read my blog: https://medium.com/#magenta2127/how-does-the-new-operator-work-f7eaac692026
What the different if both of them call the constructor "Array" and generate an object?
I know that we lost this if we create some object without new:
function Animal(name) {this.name = name}
var duck = Animal('duck'); // undefined
But how that works for new Array(n) and Array(n)?
There is no difference. Check this article:
You never need to use new Object() in JavaScript. Use the object
literal {} instead. Similarly, don’t use new Array(), use the array
literal [] instead. Arrays in JavaScript work nothing like the arrays
in Java, and use of the Java-like syntax will confuse you.
Do not use new Number, new String, or new Boolean. These forms produce
unnecessary object wrappers. Just use simple literals instead.
...............................
So the rule is simple: The only time we should use the new operator is
to invoke a pseudoclassical Constructor function. When calling a
Constructor function, the use of new is mandatory.
Such behaviour for Array is described in spec.
You can achive the same behaviour like this
function Animal(name) {
if(!(this instanceof Animal)) {
return new Animal(name);
}
this.name = name
}
var duck = Animal('duck'); //Animal {name: "duck"}
But a better idea would be to follow a simple code style convention that all functions starting with a captial letter are constructors and should be called with new. And set up a linter you prefer to check your code follows this rule.
JavaScript uses prototypal inheritance . When you use new command, It inherits from Object . Incase you want to inherit from an user defined object (e.g. Animal) you need to use new Animal() or without using new you can do it in following way.
// Function object acts as a combination of a prototype
// to use for the new object and a constructor function to invoke:
function Animal(name){
this.name = name
}
var inheritFrom = function(parent, prop){
// This is how you create object as a prototype of another object
// var x = {} syntax can’t do this as it always set the newly
//created object’s prototype to Object.prototype.
var obj = Object.create(parent.prototype);
// apply() method calls a function with a given this value
//and arguments provided as an array
parent.apply(obj, [prop]);
return obj
}
var duck = inheritFrom(Animal, 'duck');
console.log(duck.name); // ‘duck’
All, I found there are 3 ways to declare object in javascript.
var Waffle = {
tastes:'yummy'
};
var Waffle = function()
{
this.tastes='yummy';
};
function Waffle() {
var that = {};
that.tastes = "yummy";
return that;
}
The first way is Object literal it doesn't have a constructor.
I think the equal way to Object literal is below.
var Waffle = new Object();
Waffle.tastes = 'yummy';
(If my understanding is wrong. Please correct me.)
I want to know what the difference of these 3 ways is.
Which one is the best choice?
Thanks.
The literal notation and new Object() creates an object whose prototype is directly the Object. Also, properties and methods are attached on the instance.
/*
Object
A
| instance.__proto__
|
instance
*/
//all 3 yield the same result
var foo1 = {
bar : 'bam!'
}
var foo2 = {}
foo2.bar = 'bam!';
var foo3 = new Object();
foo3.bar = 'bam!';
The constructor approach, either the declared function or the assigned function expression approach, has an additional prototype level between the instance and the Object, which contains your prototype functions attached to the constructor's prototype. Anything attached to the constructor's prototype are shared across all instances.
/*
Object
A
| instance.__proto__.__proto__
|
constructor.prototype
A
| instance.__proto__
|
instance
*/
//these 2 are the same
//bar is attached at the instance
//function expression assigned to variable foo1
var foo1 = function(){
this.bar = 'bam!'
}
//function declaration named foo2
function foo2(){
this.bar = 'bam!'
}
//==========================================================================//
//these 2 are the same, but not the same as above
//methods live on the prototype level and are shared across instances
//function expression assigned to variable foo1
var foo1 = function(){}
//function declaration named foo2
function foo2(){}
foo1.prototype.bar = 'bam!'
foo2.prototype.bar = 'bam!'
The third approach returns a new literal. You don't get the benefits of the constructor method and prototype sharing. It's as if you just called Waffle like an ordinary function, created a literal, attached properties and methods, and returned it.
Best Choice: Depends on purpose.
Object literals:
Shorter than new Object and can attach methods/properties upon definition.
Properties/methods live on the instance, no running up the prototype chain which means faster look-up.
Unoptimized creation of objects might lead to duplicates, which waste memory. For example, creating functions per instance instead of sharing via the prototype.
Constructor:
Classical OOP style
Inheritance
Shared methods via the prototype means less memory used, as opposed to per instance methods.
Forgetting the new might have unwanted consequences, like attaching globals (if window is the context)
You can check these out in the Chrome Developer tools. Create them in the Console, and watch these expressions in the Sources tab
The first is an object literal and is the same as:
var Waffle = new Object();
Waffle.tastes = 'yummy';
which is the same as:
var Waffle = {};
Waffle.tastes = 'yummy';
but of course, their instantiations take multiple lines.
Your second and third examples are functions. Your second example is an expression, while your third example is a declaration. Here's an explanation of their differences: What is the difference between a function expression vs declaration in JavaScript?
Be careful with your second example (the expression), because in order to modify this (correctly), you need to use var waffle = new Waffle();. Since this is an expression, alternatively you could've used a declaration like:
function Waffle() {
this.tastes='yummy';
}
(to understand the main difference between that and the original, which I don't think affect many people ever, read the link I provided)
The third example is a basic function that returns a new object literal.
As for the best choice...I don't think there's a concrete, definite answer.
Your first example creates a simple, single object. You can add/change properties and methods on it. But it inherits directly from Object. I like to use this when I need one big object for holding many things, and its structure is static. To reuse this, your third example is needed.
Your second example is convenient because you can invoke a new instance, giving the effect of classic OOP...and is the one I normally use. Although the caveat is that you have to use new, otherwise the value of this will be window, which is bad. At the same time, you can modify this prototype and all instances will share it. It also gives you flexibility of having "private" variables, such as:
function Waffle() {
var testing = 0;
this.name = "A";
this.myMethod = function () {
return testing;
};
}
Your third example is similar to the second, except that you can't tap into a prototype, since you're returning an Object. It's basically using your first example, but making it reusable. It also allows for "private" variables like above, as well.
So if you're looking for a single object, use your first example. If you're looking for reusability, I'd suggest your second example, while the third is still an example. Hopefully I explained enough about each for you to determine which suits your needs.