JavaScript: Class.method vs. Class.prototype.method

Question

What is the difference between the following two declarations?

Class.method = function () { /* code */ }
Class.prototype.method = function () { /* code using this.values */ }

Is it okay to think of the first statement as a declaration of a static method, and the second statement as a declaration of an instance method?

Answer 1

Yes, the first function has no relationship with an object instance of that constructor function, you can consider it like a 'static method'.

In JavaScript functions are first-class objects, that means you can treat them just like any object, in this case, you are only adding a property to the function object.

The second function, as you are extending the constructor function prototype, it will be available to all the object instances created with the new keyword, and the context within that function (the this keyword) will refer to the actual object instance where you call it.

Consider this example:

// constructor function
function MyClass () {
  var privateVariable; // private member only available within the constructor fn

  this.privilegedMethod = function () { // it can access private members
    //..
  };
}

// A 'static method', it's just like a normal function 
// it has no relation with any 'MyClass' object instance
MyClass.staticMethod = function () {};

MyClass.prototype.publicMethod = function () {
  // the 'this' keyword refers to the object instance
  // you can access only 'privileged' and 'public' members
};

var myObj = new MyClass(); // new object instance

myObj.publicMethod();
MyClass.staticMethod();