Static variables in JavaScript

Question

How can I create static variables in Javascript?

Answer 1

If you come from a class-based, statically typed object-oriented language (like Java, C++ or C#) I assume that you are trying to create a variable or method associated to a "type" but not to an instance.

An example using a "classical" approach, with constructor functions maybe could help you to catch the concepts of basic OO JavaScript:

function MyClass () { // constructor function   var privateVariable = "foo";  // Private variable     this.publicVariable = "bar";  // Public variable     this.privilegedMethod = function () {  // Public Method     alert(privateVariable);   }; }  // Instance method will be available to all instances but only load once in memory  MyClass.prototype.publicMethod = function () {       alert(this.publicVariable); };  // Static variable shared by all instances MyClass.staticProperty = "baz";  var myInstance = new MyClass(); 

staticProperty is defined in the MyClass object (which is a function) and has nothing to do with its created instances, JavaScript treats functions as first-class objects, so being an object, you can assign properties to a function.

UPDATE: ES6 introduced the ability to declare classes through the class keyword. It is syntax sugar over the existing prototype-based inheritance.

The static keyword allows you to easily define static properties or methods in a class.

Let's see the above example implemented with ES6 classes:

class MyClass {    // class constructor, equivalent to    // the function body of a constructor    constructor() {      const privateVariable = 'private value'; // Private variable at the constructor scope      this.publicVariable = 'public value'; // Public property        this.privilegedMethod = function() {        // Public Method with access to the constructor scope variables        console.log(privateVariable);      };    }      // Prototype methods:    publicMethod() {      console.log(this.publicVariable);    }      // Static properties shared by all instances    static staticProperty = 'static value';      static staticMethod() {      console.log(this.staticProperty);    }  }    // We can add properties to the class prototype  MyClass.prototype.additionalMethod = function() {    console.log(this.publicVariable);  };    var myInstance = new MyClass();  myInstance.publicMethod();       // "public value"  myInstance.additionalMethod(); // "public value"  myInstance.privilegedMethod(); // "private value"  MyClass.staticMethod();             // "static value"