The new
operator creates an instance of a user-defined object type or of one of the built-in object types that has a constructor function.
Syntax
new constructor[([arguments])]
Parameters
constructor
- A function that specifies the type of the object instance.
arguments
- A list of values that the
constructor
will be called with.
Description
Creating a user-defined object requires two steps:
- Define the object type by writing a function.
- Create an instance of the object with
new
.
To define an object type, create a function for the object type that specifies its name and properties. An object can have a property that is itself another object. See the examples below.
When the code new foo(...)
is executed, the following things happen:
- A new object is created, inheriting from
foo.prototype
. - The constructor function
foo
is called with the specified arguments andthis
bound to the newly created object.new foo
is equivalent tonew foo()
, i.e. if no argument list is specified,foo
is called without arguments. - The object returned by the constructor function becomes the result of the whole
new
expression. If the constructor function doesn't explicitly return an object, the object created in step 1 is used instead. (Normally constructors don't return a value, but they can choose to do so if they want to override the normal object creation process.)
You can always add a property to a previously defined object. 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 definition of the Car
object type.
You can add a shared property to a previously defined object type by using the Function.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 null
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() {} car1 = new Car(); console.log(car1.color); // undefined Car.prototype.color = null; console.log(car1.color); // null car1.color = "black"; console.log(car1.color); // black
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:
var 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:
var 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:
var rand = new Person("Rand McNally", 33, "M"); var 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:
var car1 = new Car("Eagle", "Talon TSi", 1993, rand); var 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:
car2.owner.name
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 1st Edition. | Standard | Initial definition. Implemented in JavaScript 1.0 |
ECMAScript 5.1 (ECMA-262) The definition of 'The new Operator' in that specification. |
Standard | |
ECMAScript 6 (ECMA-262) The definition of 'The new Operator' in that specification. |
Release Candidate |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |