The mysterious 'this' keyword Let's Demystify it.

Introduction

The 'this' keyword is considered one of the confusing topics by JavaScript developers. Even experienced developers get it wrong sometimes.

So, should you be scared now? NO.

Developers get it wrong because they either don't know all the scenarios or by mistake sometimes.

Let's begin and cover all the scenarios where the value of this might get different.

What will we learn today?

1. this in the global scope
2. this inside functions ( all four cases).
3. this inside arrow functions.
4. Binding the value of this to a custom object.

Also, we will discuss the consequences of strict mode in all the above cases.

1. this in the global scope

In the global context, this refers to the global object. The value of the global object depends on the environment in which you are running your JavaScript code.

If you are running your JavaScript code in a browser, then the global object is the window object. And, In the NodeJS environment, the global object is called the 'global object' itself.

Let's verify it by writing some code.

console.log(this === window); 

// output
true

Notice the output is true because we run the code in the browser, that's why this is equal to the window.

2. this inside Functions

The value of this inside a function depends upon how that function is called. There are four ways to call a function. Let's look at them one by one -

I) Making a standalone call

Calling a function directly like function_name(); is termed a standalone call. In this case, the value of this inside the function is the window object.

But, If you're doing so in strict mode then the value of this becomes undefined in the function.

Let me show you both cases -

function foo(){
    console.log(this === window) // true
}

foo();

function bar(){
 'use strict'
    console.log(this) // undefined
}

bar();

Note that the value of this inside function foo is equal to the window object. But, as we have activated strict mode inside the bar, the value of this is undefined there.

II) Calling the function in the context of an object.

When a function is a property on an object then we call that method like object.function_name();. This way of calling a function is termed as calling in the context of an object.

So, When a function is called as a method on an object, then the value of this is set to the object which was used to call the method.

Let's take an example for more clarification.

const person = {
  name: "Faheem",
  sm:"Twitter",
  foo: function(){
      return this;
  }
}

console.log(person.foo() === person); // true

Notice the console prints true as expected. Method foo has been called in the context of the object person, so the value of this inside foo is the same as the object person itself.

III) Calling a function in constructor mode

When we call a function using the new keyword, It is termed as calling the function in constructor mode. When we call a function in constructor mode, the value of this is the new object being constructed.

See example below

function Student(name, roll, course){
    this.name = name;
    this.roll = roll;
    this.course = course;
}

const student1 = new Student("Vinay", 1011, ['Flutter', 'Dart']);
console.log(student1.name); // "Vinay"

Notice that we have used this to set properties on the new object being created.

IV) Calling using function_name.call()

The fourth way to call a function is using the call property of the function. In JavaScript, every function has a property named call which can be used to call the function. I know it sounds weird but this is what it is.

Let me show you how to use call property first.

function foo(){
    console.log("I'm foo");
}
foo.call();

// Output
"I'm foo"

Now, You have seen how to call a function using the call property. Are you thinking, who the hell will call a function this way if we can call directly?

Here is one reason. When we invoke a function using the property call, we can actually pass an object to the call() and inside that function, this will point to the object which was passed as the argument.

Doesn't make any sense? See the example for clarification

function foo(){
    console.log(this.name);
}
const person = {
  name:"Faheem",
  sm:"Twitter"
}
foo.call(person);

// output 
"Faheem"

Example Explanation

We have invoked foo like foo.call(person). This line calls the function foo and sets the value of this to the person object inside the function foo.

That is why this.name inside the functionfoo prints "Faheem".

So these are all four ways to call a function and their behaviors towards this.

3. this inside arrow function

The arrow functions don't have their own value of this. The value of this inside an arrow function is equivalent to the value of this in its outer environment.

Putting differently, the arrow function borrows the value of this from its parent's scope.

Let's take numerous examples to clarify this.

const obj = {
  name:"Faheem",
  sm:"Twitter",
  bio: () => this === window
}

console.log(obj.bio()); // true

Notice because bio is an arrow function, It has taken this from the parent context which is actually the global context. Hence, this in bio refers to the window object.

Another Example

const person = {
  myname:"Faheem",
  sm:"Twitter",
  bio: function(){
          setTimeout(() => {
          console.log(this.myname);
      }, 100)
  },
}

person.bio();

//output
"Faheem"

Let me explain the above code

We are calling bio in the person object context. So, the value of this inside bio is the person object itself.

We are using this not inside the bio function directly but inside the arrow function of setTimeout. The this.myname inside arrow function returns "Faheem" because the arrow function has taken this value from the outer scope, that is from the scope of the bio.

If you replace the arrow function inside setTimeout with a regular function, you will get undefined as the output.

See below

  const person = {
  myname:"Faheem",
  sm:"Twitter",
  bio: function(){
          setTimeout(function(){
          console.log(this.myname);
      }, 100)
  },
}

person.bio();
//Output
undefined

The function inside setTimeout is not an arrow function. So it tries accessing its own this unlike the arrow function which accesses this from the parent. That is why this.myname returns undefined, not 'Faheem'.

4. Binding the value of this to a customer object.

We can bind the value of this dynamically using call (we have seen this), apply, and bind methods. If you don't know about these methods , you can Click👆

Summary

• The value of this is determined by the context in which it is accessed.
• First, check how a function is being called and then determine what could be the value of this inside that function.
• Arrow functions don't have it's own this. It borrows the value of this from the parent's scope.