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[([arguments])]
构造函数
A class or function that specifies the type of the object instance.
arguments
构造函数
will be called with.
new
keyword does the following things:
this
context;
this
if the function doesn't return an object.
Creating a user-defined object requires two steps:
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:
Foo
.prototype
.
Foo
is called with the specified arguments, and with
this
bound to the newly created object.
new
Foo
相当于
new
Foo
()
, i.e. if no argument list is specified,
Foo
is called without arguments.
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
"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() {}
car1 = new Car();
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'
If you didn't write the
new
operator,
the Constructor Function would be invoked like any Regular Function,
without creating an Object.
In this case, the value of
this
is also different.
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
。例如：
var 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:
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
| 规范 |
|---|
|
ECMAScript (ECMA-262)
The definition of 'The new Operator' in that specification. |
| Desktop | Mobile | Server | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
new
|
Chrome 1 | Edge 12 | Firefox 1 | IE 3 | Opera 3 | Safari 1 | WebView Android 1 | Chrome Android 18 | Firefox Android 4 | Opera Android 10.1 | Safari iOS 1 | Samsung Internet Android 1.0 | nodejs 0.1.100 |
完整支持