THIS IN JAVASCRIPT

Javascript this keyword is the most widely used keyword and still quite confusing for beginners. this points to a particular object in JavaScript and what is that object mostly depends on how a function which includes this is called.

The function's this keyword behaves a little differently in JavaScript than in other languages. Understanding this very important to learn advanced concepts in JavaScript.

In this tutorial, you will understand this keyword and its use in different cases in detail.

• What is this?
• this outside function (Global)
• this in function
  • this in function (strict mode)
• this in arrow function
• this in method
• this in event handler
  • this in inline event handler
  • this in event handler attached using HTML attribute
• Refer this to desired object
  • Set this using call and apply method
  • Binding this using bind() method

What is this keyword in JavaScript?

Every javascript function has reference to the object which called the function, that reference is known as this.

In JavaScript, the value of this stores current execution context of the program so when this is used inside a function then the value of this is determined by how a function is called, which is a runtime binding of this.

alert(this); // refer to global object -> Window
let user = {
  firstName: "John",
  lastName: "Smith",
  age: 24,
  intro: function () {
    return (this.firstName + " " + this.lastName + " has age = " + this.age);
    // here this refers to user object
  }
}
console.log(user.intro());

The this keyword has different value depending on where it is used:

• Alone or outside a function, this refers to the global object.
• In function, this depends on how the function is called however by default it refers to the global object.
• In normal function in strict mode, this is undefined.
• In a method, this refers to an owner object.
• In an event, this refers to an element that received the event.

this Outside function - Global context

this used outside any function and object methods has a global scope and refers to the window object.

this outside function refers to the global object even in strict mode.

this.user = "John";
console.log(window.user); // refers to window
num = 20;
console.log(num);
console.log(this === window); // true

Also in strict mode, this outside the function refers to Global object ([object Window]).

"use strict";
user = "John";
console.log(window.user); // refers to window
num = 10;
console.log(num);
console.log(this === window); // true

this in JavaScript function

The value of this inside a function depends on how the function is called.

this in function by default refers to Global object ([object Window]) in non-strict mode.

user = "John";
function foo(){
  var user = "Herry";
  console.log(user); // Herry
  console.log(this.user); // John
  // this in function refer Global object
}
foo();

this in JavaScript function - strict mode

In strict mode, this in function is undefined because in strict mode JavaScript does not allow default binding.

"use strict"
function foo() {
  console.log(this); // undefined
}
foo();

this in arrow function

Arrow function doesn't have their own this. It takes the reference to this from outer normal function.

In global code, this in arrow function refers to the global object.

Let's see an example to understand.

name = "John"
let user = {
  name: "Herry",
  showName: () => console.log(this.name), // this from Window (global)
  sayHi: function () {
    let fxn = () => console.log(this.name); // this from outer normal function -> user
    fxn();
  }
};
user.showName()
user.sayHi();

The 'name' is a global and a local variable in the object. When arrow function uses this in 'showName' function then it refers to outer scope (Global) which is 'John' and when the 'fxn' run then it take reference of this from the outer normal function which is 'sayHi'.

this in a Method

this in an object method refers to that object.

You can use this within an object method to represent object and object entities.

let user = {
  firstName: "John",
  lastName: "Smith",
  age: 24,
  intro: function(){
    return this.firstName + " " + this.lastName + " has age = " + this.age;
  }
}
console.log(user.intro());
// this in method refers to the object

this in Event Handler

this in a function which is used as an event handler for an event refers to the element which receives the event.

It is recommended to use addEventListener() method to set the event handler because some browser does not allow this convention.

Here in the example below, we set an event listener function using the addEventListener method. When you use this in the event handler function then it points to the element which receives the event.

<button id="btn">Click Me!</button>

<script>
  const button = document.getElementById("btn");
  button.addEventListener("click", change);
  function change() {
    this.innerHTML = "Button Clicked!"
  }
</script>

this in inline event handler

Also in the inline event handler, this refers to the element which receives the event.

<button onclick="this.innerHTML = 'Button Clicked!'">Show this!</button>

this in event handler attached using HTML attribute

You saw in the above example that when the event handler is added to the element using addEventHandler method then this represents the element itself however when you add event listener using HTML attribute then this in the function does not represent the element but it represents the Global object.

See in the example below.

<button onclick="ShowThis()">Show this!</button>

<script>
  function ShowThis() {
    console.log(this)
  }
</script>

But, if you want to use this which points to the element in this case then pass this as an argument in the function and use it in the event handler.

<button onclick="change(this)">Button</button>

<script>
  function change(thisOfElement) {
    thisOfElement.innerHTML = "Button clicked!"
  }
</script>

Refer this to desired object

As you have seen as far that a function's this refers to a global object but you can make to refer to a specific object for a certain call or for always.

You may be knowing about the call(), apply() and bind(). Using these 3 method you can set the context of this inside a function irrespective of whether that function is being called in the global scope or as an object's method.

Let's see in detail with example.

Set this using call and apply method

The call and apply method executes the function immediately after setting this of another object to the function.

To set the desired this, pass the reference of this as the 1st parameter in the call and apply method and pass functions parameter and next you can pass the arguments of the function.

The only difference between the call and the apply is that function's argument is passed individually in call method but passed as an array in apply method.

name = "Peter";
age = 15;
let user1 = { name: "John", age: 25 }
let user2 = { name: "Herry", age: 30 }
function intro() {
  console.log("My name is " + this.name + " age, " + this.age + " years")
}
intro(); // Global this
intro.call(user1); // this of user1
intro.call(user2); // this of user2
intro.apply(user1); // this of user1
intro.apply(user2); // this of user2

As you can see in the above example, when function 'info' is called normally then this refers to global object while when called using call and apply its this refers to the passed object.

Binding this using bind() method

bind() method bind this of another object and returns a new function.

Unlike call and apply methods, bind does not invoke immediately, you can call it later which makes it suitable to be used as callback functions.

name = "Peter";
age = 15;
let user1 = { name: "John", age: 25 }
let user2 = { name: "Herry", age: 30 }
function intro() {
  console.log("My name is " + this.name + " age, " + this.age + " years")
}
var user1Intro = intro.bind(user1); // bind this of user2
var user2Intro = intro.bind(user2); // bind this of user2
intro(); // Global this
user1Intro();
user2Intro();

// or directly execute
intro.bind(user1)();

As you can see in the above example, the bind method binds another object to 'intro' function and return a new function which you can store to some variable or invoke directly.

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