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"