Javascript create objects with new

JavaScript Objects

In JavaScript, objects are king. If you understand objects, you understand JavaScript.

In JavaScript, almost «everything» is an object.

• Booleans can be objects (if defined with the new keyword)
• Numbers can be objects (if defined with the new keyword)
• Strings can be objects (if defined with the new keyword)
• Dates are always objects
• Maths are always objects
• Regular expressions are always objects
• Arrays are always objects
• Functions are always objects
• Objects are always objects

All JavaScript values, except primitives, are objects.

JavaScript Primitives

A primitive value is a value that has no properties or methods.

3.14 is a primitive value

A primitive data type is data that has a primitive value.

JavaScript defines 7 types of primitive data types:

Examples

• string
• number
• boolean
• null
• undefined
• symbol
• bigint

Immutable

Primitive values are immutable (they are hardcoded and cannot be changed).

if x = 3.14, you can change the value of x, but you cannot change the value of 3.14.

Value	Type	Comment
«Hello»	string	«Hello» is always «Hello»
3.14	number	3.14 is always 3.14
true	boolean	true is always true
false	boolean	false is always false
null	null (object)	null is always null
undefined	undefined	undefined is always undefined

Objects are Variables

JavaScript variables can contain single values:

Example

JavaScript variables can also contain many values.

Objects are variables too. But objects can contain many values.

Object values are written as name : value pairs (name and value separated by a colon).

Example

A JavaScript object is a collection of named values

It is a common practice to declare objects with the const keyword.

Example

Object Properties

The named values, in JavaScript objects, are called properties.

Property	Value
firstName	John
lastName	Doe
age	50
eyeColor	blue

Objects written as name value pairs are similar to:

• Associative arrays in PHP
• Dictionaries in Python
• Hash tables in C
• Hash maps in Java
• Hashes in Ruby and Perl

Object Methods

Methods are actions that can be performed on objects.

Object properties can be both primitive values, other objects, and functions.

An object method is an object property containing a function definition.

Property	Value
firstName	John
lastName	Doe
age	50
eyeColor	blue
fullName	function()

JavaScript objects are containers for named values, called properties and methods.

You will learn more about methods in the next chapters.

Creating a JavaScript Object

With JavaScript, you can define and create your own objects.

There are different ways to create new objects:

• Create a single object, using an object literal.
• Create a single object, with the keyword new .
• Define an object constructor, and then create objects of the constructed type.
• Create an object using Object.create() .

Using an Object Literal

This is the easiest way to create a JavaScript Object.

Using an object literal, you both define and create an object in one statement.

An object literal is a list of name:value pairs (like age:50) inside curly braces <>.

The following example creates a new JavaScript object with four properties:

Example

Spaces and line breaks are not important. An object definition can span multiple lines:

Example

This example creates an empty JavaScript object, and then adds 4 properties:

Example

const person = <>;
person.firstName = «John»;
person.lastName = «Doe»;
person.age = 50;
person.eyeColor = «blue»;

Using the JavaScript Keyword new

The following example create a new JavaScript object using new Object() , and then adds 4 properties:

Example

const person = new Object();
person.firstName = «John»;
person.lastName = «Doe»;
person.age = 50;
person.eyeColor = «blue»;

The examples above do exactly the same.

But there is no need to use new Object() .

For readability, simplicity and execution speed, use the object literal method.

JavaScript Objects are Mutable

Objects are mutable: They are addressed by reference, not by value.

If person is an object, the following statement will not create a copy of person:

The object x is not a copy of person. It is person. Both x and person are the same object.

Any changes to x will also change person, because x and person are the same object.

Example

const person = <
firstName:»John»,
lastName:»Doe»,
age:50, eyeColor:»blue»
>

const x = person;
x.age = 10; // Will change both x.age and person.age

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new operator

The new operator lets developers create an instance of a user-defined object type or of one of the built-in object types that has a constructor function.

Try it

Syntax

new constructor new constructor() new constructor(arg1) new constructor(arg1, arg2) new constructor(arg1, arg2, /* …, */ argN) 

Parameters

A class or function that specifies the type of the object instance.

A list of values that the constructor will be called with. new Foo is equivalent to new Foo() , i.e. if no argument list is specified, Foo is called without arguments.

Description

When a function is called with the new keyword, the function will be used as a constructor. new will do the following things:

1. Creates a blank, plain JavaScript object. For convenience, let’s call it newInstance .
2. Points newInstance ‘s [[Prototype]] to the constructor function’s prototype property, if the prototype is an Object . Otherwise, newInstance stays as a plain object with Object.prototype as its [[Prototype]].

Note: Properties/objects added to the constructor function’s prototype property are therefore accessible to all instances created from the constructor function.

Classes can only be instantiated with the new operator — attempting to call a class without new will throw a TypeError .

Creating an object with a user-defined constructor function requires two steps:

Define the object type by writing a function that specifies its name and properties. For example, a constructor function to create an object Foo might look like this:

function Foo(bar1, bar2)  this.bar1 = bar1; this.bar2 = bar2; > 

const myFoo = new Foo("Bar 1", 2021); 

Note: An object can have a property that is itself another object. See the examples below.

You can always add a property to a previously defined object instance. For example, the statement car1.color = «black» adds a property color to car1 , and assigns it a value of «black» .

However, this does not affect any other objects. To add the new property to all objects of the same type, you must add the property to the constructor’s prototype property. This defines a property that is shared by all objects created with that function, rather than by just one instance of the object type. The following code adds a color property with value «original color» to all objects of type Car , and then overwrites that value with the string «black» only in the instance object car1 . For more information, see prototype.

function Car() > const car1 = new Car(); const car2 = new Car(); console.log(car1.color); // undefined Car.prototype.color = "original color"; console.log(car1.color); // 'original color' car1.color = "black"; console.log(car1.color); // 'black' console.log(Object.getPrototypeOf(car1).color); // 'original color' console.log(Object.getPrototypeOf(car2).color); // 'original color' console.log(car1.color); // 'black' console.log(car2.color); // 'original color' 

Note: While the constructor function can be invoked like any regular function (i.e. without the new operator), in this case a new object is not created and the value of this is also different.

A function can know whether it is invoked with new by checking new.target . new.target is only undefined when the function is invoked without new . For example, you can have a function that behaves differently when it’s called versus when it’s constructed:

function Car(color)  if (!new.target)  // Called as function. return `$color> car`; > // Called with new. this.color = color; > const a = Car("red"); // a is "red car" const b = new Car("red"); // b is `Car < color: "red" >` 

Prior to ES6, which introduced classes, most JavaScript built-ins are both callable and constructible, although many of them exhibit different behaviors. To name a few:

• Array() , Error() , and Function() behave the same when called as a function or a constructor.
• Boolean() , Number() , and String() coerce their argument to the respective primitive type when called, and return wrapper objects when constructed.
• Date() returns a string representing the current date when called, equivalent to new Date().toString() .

After ES6, the language is stricter about which are constructors and which are functions. For example:

• Symbol() and BigInt() can only be called without new . Attempting to construct them will throw a TypeError .
• Proxy and Map can only be constructed with new . Attempting to call them will throw a TypeError .

Examples

Object type and object instance

Suppose you want to create an object type for cars. You want this type of object to be called Car , and you want it to have properties for make, model, and year. To do this, you would write the following function:

function Car(make, model, year)  this.make = make; this.model = model; this.year = year; > 

Now you can create an object called myCar as follows:

const myCar = new Car("Eagle", "Talon TSi", 1993); 

This statement creates myCar and assigns it the specified values for its properties. Then the value of myCar.make is the string «Eagle», myCar.year is the integer 1993, and so on.

You can create any number of car objects by calls to new . For example:

const kensCar = new Car("Nissan", "300ZX", 1992); 

Object property that is itself another object

Suppose you define an object called Person as follows:

function Person(name, age, sex)  this.name = name; this.age = age; this.sex = sex; > 

And then instantiate two new Person objects as follows:

const rand = new Person("Rand McNally", 33, "M"); const ken = new Person("Ken Jones", 39, "M"); 

Then you can rewrite the definition of Car to include an owner property that takes a Person object, as follows:

function Car(make, model, year, owner)  this.make = make; this.model = model; this.year = year; this.owner = owner; > 

To instantiate the new objects, you then use the following:

const car1 = new Car("Eagle", "Talon TSi", 1993, rand); const car2 = new Car("Nissan", "300ZX", 1992, ken); 

Instead of passing a literal string or integer value when creating the new objects, the above statements pass the objects rand and ken as the parameters for the owners. To find out the name of the owner of car2 , you can access the following property:

Using new with classes

class Person  constructor(name)  this.name = name; > greet()  console.log(`Hello, my name is $this.name>`); > > const p = new Person("Caroline"); p.greet(); // Hello, my name is Caroline 

Specifications

Browser compatibility

BCD tables only load in the browser

See also

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This page was last modified on Feb 21, 2023 by MDN contributors.

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