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.
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.
new constructor new constructor() new constructor(arg1) new constructor(arg1, arg2) new constructor(arg1, arg2, /* …, */ argN)
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 to new Foo()
, i.e. if no argument list is specified, Foo
is called without arguments.
When a function is called with the new
keyword, the function will be used as a constructor. new
will do the following things:
newInstance
.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.
newInstance
as the this
context (i.e. all references to this
in the constructor function now refer to newInstance
).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:
Foo
might look like this: function Foo(bar1, bar2) { this.bar1 = bar1; this.bar2 = bar2; }
new
. 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:
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);
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:
car2.owner.name;
new
with classesclass 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
Specification |
---|
ECMAScript Language Specification # sec-new-operator |
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Chrome | Edge | Firefox | Opera | Safari | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | WebView Android | Deno | Node.js | ||
new |
1 | 12 | 1 | 3 | 1 | 18 | 4 | 10.1 | 1 | 1.0 | 4.4 | 1.0 | 0.10.0 |
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https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new