static - JavaScript 编辑

The static keyword defines a static method or property for a class. Neither static methods nor static properties can be called on instances of the class. Instead, they're called on the class itself. Static methods are often utility functions, such as functions to create or clone objects, whereas static properties are useful for caches, fixed-configuration, or any other data you don't need to be replicated across instances.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

static methodName() { ... }
static propertyName [= value];

Examples

Using static members in classes

The following example demonstrates several things:

1. How a static member (method or property) is defined on a class.
2. That a class with a static member can be sub-classed.
3. How a static member can and cannot be called.

class Triple {
  static customName = 'Tripler';
  static description = 'I triple any number you provide';
  static triple(n = 1) {
    return n * 3;
  }
}

class BiggerTriple extends Triple {
  static longDescription;
  static description = 'I square the triple of any number you provide';
  static triple(n) {
    return super.triple(n) * super.triple(n);
  }
}

console.log(Triple.description);   // 'I triple any number you provide'
console.log(Triple.triple());      // 3
console.log(Triple.triple(6));     // 18

var tp = new Triple();

console.log(BiggerTriple.triple(3));        // 81 (not affected by parent's instantiation)
console.log(BiggerTriple.description);      // 'I square the triple of any number you provide'
console.log(BiggerTriple.longDescription);  // undefined
console.log(BiggerTriple.customName);       // 'Tripler'

console.log(tp.triple());         // 'tp.triple is not a function'.

Calling static members from another static method

In order to call a static method or property within another static method of the same class, you can use the this keyword.

class StaticMethodCall {
  static staticProperty = 'static property';
  static staticMethod() {
    return 'Static method and ' + this.staticProperty + ' has been called';
  }
  static anotherStaticMethod() {
    return this.staticMethod() + ' from another static method';
  }
}
StaticMethodCall.staticMethod();
// 'Static method and static property has been called'

StaticMethodCall.anotherStaticMethod();
// 'Static method and static property has been called from another static method'

Calling static members from a class constructor and other methods

Static members are not directly accessible using the this keyword from non-static methods. You need to call them using the class name: CLASSNAME.STATIC_METHOD_NAME() / CLASSNAME.STATIC_PROPERTY_NAME or by calling the method as a property of the constructor: this.constructor.STATIC_METHOD_NAME() / this.constructor.STATIC_PROPERTY_NAME

class StaticMethodCall {
  constructor() {
    console.log(StaticMethodCall.staticProperty); // 'static property'
    console.log(this.constructor.staticProperty); // 'static property'
    console.log(StaticMethodCall.staticMethod()); // 'static method has been called.'
    console.log(this.constructor.staticMethod()); // 'static method has been called.'
  }

  static staticProperty = 'static property';
  static staticMethod() {
    return 'static method has been called.';
  }
}

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'Class definitions' in that specification.

Browser compatibility

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See also

• class expression
• class declaration
• Classes