构造函数
method is a special method of a
class
for creating and initializing an object of that class.
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constructor([arguments]) { ... }
A constructor enables you to provide any custom initialization that must be done before any other methods can be called on an instantiated object.
class Person {
constructor(name) {
this.name = name;
}
introduce() {
console.log(`Hello, my name is ${this.name}`);
}
}
const otto = new Person('Otto');
otto.introduce();
If you don't provide your own constructor, then a default constructor will be supplied for you. If your class is a base class, the default constructor is empty:
constructor() {}
If your class is a derived class, the default constructor calls the parent constructor, passing along any arguments that were provided:
constructor(...args) {
super(...args);
}
That enables code like this to work:
class ValidationError extends Error {
printCustomerMessage() {
return `Validation failed :-( (details: ${this.message})`;
}
}
try {
throw new ValidationError("Not a valid phone number");
} catch (error) {
if (error instanceof ValidationError) {
console.log(error.name); // This is Error instead of ValidationError!
console.log(error.printCustomerMessage());
} else {
console.log('Unknown error', error);
throw error;
}
}
ValidationError
class doesn't need an explicit constructor, because it doesn't need to do any custom initialization. The default constructor then takes care of initializing the parent
Error
from the argument it is given.
However, if you provide your own constructor, and your class derives from some parent class, then you must explicitly call the parent class constructor using
super
。例如：
class ValidationError extends Error {
constructor(message) {
super(message); // call parent class constructor
this.name = 'ValidationError';
this.code = '42';
}
printCustomerMessage() {
return `Validation failed :-( (details: ${this.message}, code: ${this.code})`;
}
}
try {
throw new ValidationError("Not a valid phone number");
} catch (error) {
if (error instanceof ValidationError) {
console.log(error.name); // Now this is ValidationError!
console.log(error.printCustomerMessage());
} else {
console.log('Unknown error', error);
throw error;
}
}
There can be only one special method with the name "
构造函数
" in a class. Having more than one occurrence of a
构造函数
method in a class will throw a
SyntaxError
error.
构造函数
方法
This code snippet is taken from the classes sample ( live demo ).
class Square extends Polygon {
constructor(length) {
// Here, it calls the parent class' constructor with lengths
// provided for the Polygon's width and height
super(length, length);
// NOTE: In derived classes, `super()` must be called before you
// can use `this`. Leaving this out will cause a ReferenceError.
this.name = 'Square';
}
get area() {
return this.height * this.width;
}
set area(value) {
this.height = value**0.5;
this.width = value**0.5;
}
}
Here the prototype of
Square
class is changed—but the constructor of its base class
Polygon
is still called when a new instance of a square is created.
class Polygon {
constructor() {
this.name = "Polygon";
}
}
class Square extends Polygon {
constructor() {
super();
}
}
class Rectangle {}
Object.setPrototypeOf(Square.prototype, Rectangle.prototype);
console.log(Object.getPrototypeOf(Square.prototype) === Polygon.prototype); //false
console.log(Object.getPrototypeOf(Square.prototype) === Rectangle.prototype); //true
let newInstance = new Square();
console.log(newInstance.name); //Polygon
| 规范 |
|---|
|
ECMAScript (ECMA-262)
The definition of 'Constructor Method' in that specification. |
| Desktop | Mobile | Server | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
构造函数
|
Chrome
49
|
Edge 13 | Firefox 45 | IE No |
Opera
36
|
Safari 9 |
WebView Android
49
|
Chrome Android
49
|
Firefox Android 45 |
Opera Android
36
|
Safari iOS 9 |
Samsung Internet Android
5.0
|
nodejs
6.0.0
|
完整支持
不支持
见实现注意事项。
用户必须明确启用此特征。