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
js
new constructor
new constructor()
new constructor(arg1)
new constructor(arg1, arg2)
new constructor(arg1, arg2, /* …, */ argN)
Parameters
constructor
-
A class or function that specifies the type of the object instance.
arg1
,arg2
, …,argN
-
A list of values that the
constructor
will be called with.new Foo
is equivalent tonew 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:
- Creates a blank, plain JavaScript object. For convenience, let's call it
newInstance
. - Points
newInstance
's [[Prototype]] to the constructor function'sprototype
property, if theprototype
is anObject
. Otherwise,newInstance
stays as a plain object withObject.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. - Executes the constructor function with the given arguments, binding
newInstance
as thethis
context (i.e. all references tothis
in the constructor function now refer tonewInstance
). - If the constructor function returns a non-primitive, this return value becomes the result of the whole
new
expression. Otherwise, if the constructor function doesn't return anything or returns a primitive,newInstance
is returned instead. (Normally constructors don't return a value, but they can choose to do so to override the normal object creation process.)
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:js
function Foo(bar1, bar2) { this.bar1 = bar1; this.bar2 = bar2; }
- Create an instance of the object with
new
.js
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.
js
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:
js
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()
, andFunction()
behave the same when called as a function or a constructor.Boolean()
,Number()
, andString()
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 tonew Date().toString()
.
After ES6, the language is stricter about which are constructors and which are functions. For example:
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:
js
function Car(make, model, year) {
this.make = make;
this.model = model;
this.year = year;
}
Now you can create an object called myCar
as follows:
js
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:
js
const kensCar = new Car("Nissan", "300ZX", 1992);
Object property that is itself another object
Suppose you define an object called Person
as follows:
js
function Person(name, age, sex) {
this.name = name;
this.age = age;
this.sex = sex;
}
And then instantiate two new Person
objects as follows:
js
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:
js
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:
js
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:
js
car2.owner.name;
Using new
with classes
js
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
Specification |
---|
ECMAScript Language Specification # sec-new-operator |
Browser compatibility
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