static

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.

Note that the examples throughout this article use public class fields (including static public class fields), which are not yet part of the ECMAScript specification, but are instead specified in a Public and private instance fields proposal at TC39.

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

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 calculate(n = 1) {
    return n * 3;
  }
}

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

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

const tp = new Triple();

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

// This throws because calculate() is a static member, not an instance member.
console.log(tp.calculate());                // 'tp.calculate is not a function'

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'

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.';
  }
}

• class expression
• class declaration
• Classes