static

JavaScript static Keyword Syntax

The static keyword marks fields, methods, and initialization blocks in a class body:

class <Name> {
  static <field> [= <value>];
  static <method>([<parameters>]) {
    <statements>
  }
  static {
    <statements>
  }
}

The static prefix also combines with private #names, get, set, async, and generator methods. A static block can’t contain await or arguments, and its declarations stay local to the block.

JavaScript static Keyword Examples

Static members belong to the Temperature class itself, not to its instances:

> class Temperature {
…   static unit = "Celsius";
…   static fromFahrenheit(degrees) {
…     return new Temperature(((degrees - 32) * 5) / 9);
…   }
…   constructor(degrees) {
…     this.degrees = degrees;
…   }
… }
undefined

> Temperature.fromFahrenheit(212)
Temperature { degrees: 100 }

> Temperature.unit
'Celsius'
> new Temperature(20).unit
undefined

The static method works as an alternative constructor, and the static field holds data for the whole class. An instance reads .unit as undefined, because static members live on the class, not on the Temperature.prototype object that instances inherit from.

A static initialization block runs setup code that a single field initializer can’t express:

> class Weekday {
…   static names = ["Mon", "Tue", "Wed"];
…   static indexOf = new Map();
…   static {
…     this.names.forEach((name, index) => this.indexOf.set(name, index));
…   }
… }
undefined

> Weekday.indexOf.get("Tue")
1

The block runs after the fields above it, so both already exist.

JavaScript static Keyword Use Cases

• Defining factory methods and alternative constructors
• Grouping utility functions that don’t need an instance
• Storing constants, caches, or counters shared by every instance
• Running one-time class setup in a static initialization block