Static variables in JavaScript

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 instancesMyClass.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 prototypeMyClass.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"