Object methods, "this"

Objects are usually created to represent entities of the real world, like users, orders and so on:

          let user = {
  name: "John",
  age: 30
};
        

And, in the real world, a user can act: select something from the shopping cart, login, logout etc.

Actions are represented in JavaScript by functions in properties.

Method examples

For a start, let’s teach the user to say hello:

let user = {
  name: "John",
  age: 30
};

user.sayHi = function() {
  alert("Hello!");
};

user.sayHi(); // Hello!

Here we’ve just used a Function Expression to create a function and assign it to the property user.sayHi of the object.

Then we can call it as user.sayHi(). The user can now speak!

A function that is a property of an object is called its method.

So, here we’ve got a method sayHi of the object user.

Of course, we could use a pre-declared function as a method, like this:

let user = {
  // ...
};

// first, declare
function sayHi() {
  alert("Hello!");
}

// then add as a method
user.sayHi = sayHi;

user.sayHi(); // Hello!

When we write our code using objects to represent entities, that’s called object-oriented programming, in short: “OOP”.

OOP is a big thing, an interesting science of its own. How to choose the right entities? How to organize the interaction between them? That’s architecture, and there are great books on that topic, like “Design Patterns: Elements of Reusable Object-Oriented Software” by E. Gamma, R. Helm, R. Johnson, J. Vissides or “Object-Oriented Analysis and Design with Applications” by G. Booch, and more.

Method shorthand

There exists a shorter syntax for methods in an object literal:

          // these objects do the same

user = {
  sayHi: function() {
    alert("Hello");
  }
};

// method shorthand looks better, right?
user = {
  sayHi() { // same as "sayHi: function(){...}"
    alert("Hello");
  }
};
        

As demonstrated, we can omit "function" and just write sayHi().

To tell the truth, the notations are not fully identical. There are subtle differences related to object inheritance (to be covered later), but for now they do not matter. In almost all cases, the shorter syntax is preferred.

“this” in methods

It’s common that an object method needs to access the information stored in the object to do its job.

For instance, the code inside user.sayHi() may need the name of the user.

To access the object, a method can use the this keyword.

The value of this is the object “before dot”, the one used to call the method.

For instance:

let user = {
  name: "John",
  age: 30,

  sayHi() {
    // "this" is the "current object"
    alert(this.name);
  }

};

user.sayHi(); // John

Here during the execution of user.sayHi(), the value of