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!

Object-oriented programming

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 this will be user.

Technically, it’s also possible to access the object without this, by referencing it via the outer variable:

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

  sayHi() {
    alert(user.name); // "user" instead of "this"
  }

};

…But such code is unreliable. If we decide to copy user to another variable, e.g. admin = user and overwrite user with something else, then it will access the wrong object.

That’s demonstrated below:

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

  sayHi() {
    alert( user.name ); // leads to an error
  }

};


let admin = user;
user = null; // overwrite to make things obvious

admin.sayHi(); // TypeError: Cannot read property 'name' of null

If we used this.name instead of user.name inside the alert, then the code would work.

“this” is not bound

In JavaScript, keyword this behaves unlike most other programming languages. It can be used in any function, even if it’s not a method of an object.

There’s no syntax error in the following example:

function sayHi() {
  alert( this.name );
}

The value of this is evaluated during the run-time, depending on the context.

For instance, here the same function is assigned to two different objects and has different “this” in the calls:

let user = { name: "John" };
let admin = { name: "Admin" };

function sayHi() {
  alert( this.name );
}

// use the same function in two objects
user.f = sayHi;
admin.f = sayHi;

// these calls have different this
// "this" inside the function is the object "before the dot"
user.f(); // John  (this == user)
admin.f(); // Admin  (this == admin)

admin['f'](); // Admin (dot or square brackets access the method – doesn't matter)

The rule is simple: if obj.f() is called, then this is obj during the call of f. So it’s either user or admin in the example above.

Calling without an object: this == undefined

We can even call the function without an object at all:

function sayHi() {
  alert(this);
}

sayHi(); // undefined

In this case this is undefined in strict mode. If we try to access this.name, there will be an error.

In non-strict mode the value of this in such case will be the global object (window in a browser, we’ll get to it later in the chapter Global object). This is a historical behavior that "use strict" fixes.

Usually such call is a programming error. If there’s this inside a function, it expects to be called in an object context.

The consequences of unbound this

If you come from another programming language, then you are probably used to the idea of a “bound this”, where methods defined in an object always have this referencing that object.

In JavaScript this is “free”, its value is evaluated at call-time and does not depend on where the method was declared, but rather on what object is “before the dot”.

The concept of run-time evaluated this has both pluses and minuses. On the one hand, a function can be reused for different objects. On the other hand, the greater flexibility creates more possibilities for mistakes.

Here our position is not to judge whether this language design decision is good or bad. We’ll understand how to work with it, how to get benefits and avoid problems.

Arrow functions have no “this”

Arrow functions are special: they don’t have their “own” this. If we reference this from such a function, it’s taken from the outer “normal” function.

For instance, here arrow() uses this from the outer user.sayHi() method:

let user = {
  firstName: "Ilya",
  sayHi() {
    let arrow = () => alert(this.firstName);
    arrow();
  }
};

user.sayHi(); // Ilya

That’s a special feature of arrow functions, it’s useful when we actually do not want to have a separate this, but rather to take it from the outer context. Later in the chapter Arrow functions revisited we’ll go more deeply into arrow functions.

Common mistakes

• Skipping the small examples and then missing the exact rule that Object methods, "this" depends on.
• Copying code without changing one value at a time to see which part controls the result.
• Treating similar-looking syntax or APIs as interchangeable before checking their edge cases.

Summary

• Functions that are stored in object properties are called “methods”.
• Methods allow objects to “act” like object.doSomething().
• Methods can reference the object as this.

The value of this is defined at run-time.

• When a function is declared, it may use this, but that this has no value until the function is called.
• A function can be copied between objects.
• When a function is called in the “method” syntax: object.method(), the value of this during the call is object.

Please note that arrow functions are special: they have no this. When this is accessed inside an arrow function, it is taken from outside.

Practice

• Rewrite one example from this lesson without looking at the original, then run it and compare the result.
• Change one input, operator, method call, or option in a code sample and predict what will happen before running it.
• Explain Object methods, "this" in your own words as if you were reviewing it with another learner.

Keep learning

Continue with Constructor, operator "new" when you are ready for the next lesson.