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"