In simple terms, this keyword in JavaScript refers to an object upon which a function is invoked. This means that this can only be used in a function, or globally.
this is used to point to an instance of an object from its own constructor or its method and also to keep track of the execution context (lexical scope) - which is often based on where the function is called from.
function myself() {
this.name = "Prabesh";
console.log(this); // OUTPUT: myself {"name": "Prabesh"}
}
let intro = new myself();In the example above, myself is used as constructor for the object of type myself. Here, this reference points to itself i.e. myself {name: "Prabesh"}.
When we use the new keyword to create an instance of the myself() function, it creates new object and sets this to refer to that object. In this case, the name property is set to the new object created by the new operator.
So, when we create a new instance of myself() using let intro = new myself(), the name property is set on the newly created object, and this refers to that object. Therefore, intro will refer to an object with a name property of "Prabesh".
When used in either of function or globally, this refers to the window object.
function foo() {
console.log(this);
}
foo(); // OUTPUT : WindowHere, when we call the foo() function without new keyword, this refers to the global object (which could be window in browser or global in Node.js).
However, when we want to use the function as constructor of an object (create an object of type foo), this keyword changes its context to the instance of bar object itself and no longer global object.
function foo() {
console.log(this);
}
let bar = new foo(); //Output: foo{}
typeof bar; //Ouput: 'object'When a function is a method of an object, then this refers to object that the method is a member of. This allows us to access and manipulate the properties of the object within the method.
Example:
const user = {
name: "Prabesh",
age: 23,
greet: function () {
console.log(`Hello, my name is ${this.name}`);
},
};
user.greet(); // Output: "Hello, my name is Prabesh"In the above example, user is an object that has a greet() method. When we call user.greet(), this refers to the user object. Hence, within the greet() method, we can access the name property of user object using this.name.
The value of this inside handler function may not be what we expect it to be. The value of this inside an event handler function is determined by how the function is called. In most cases, this will refer to DOM element that triggered the event.
For example, if you have a button with an onClick attribute that calls a function, the value of this inside that function will be the button element. We can use this keyword to access properties and methods of the object that the event is asscoiated with. For example, we can use this.textContent property to get the text content of the button object.
<button id="myButton">Click me</button>
<script>
function myFunction() {
console.log(this); // logs the button element
// Output: <button id="myButton">Click me</button>
}
// Attach the event handler to the button's click event.
document.getElementById("myButton").addEventListener("click", myFunction);
</script>When the button is clicked, the myClickHandler() function will be called. The this keyword will refer to the button object, so you can use the this keyword to access and modify the button's properties and methods.
Now, let's consider annther example:
function cookFood(dish) {
this.dish = dish;
this.time = time;
function setTime(sec) {
setTimeout(function () {
console.log(this.dish + " cooked for " + sec + " seconds");
}, sec * 1000);
}
}
let curry = new cookFood("Curry"); // this.dish = "Curry"
curry.setTime(3); // Output: undefined cooked for 2 secondsSo, what happened here? We have set this.dish="Curry" when constructing the cookFood object but we get "undefined cooked for 2 seconds" when we expected the output to be "Curry cooked for 2 seconds".
Here, when we call the setTimeout function, it creates a nre execution context for the anonymous function that we pass as an argument, which has its own this context. Hence, when the anonymous function executes, this.dish will be undefined because this refers to the global object, not the cookFood object we created.
This is where arrow functions comes into rescue.
Arrow functions help us write more concise and efficient code. Arrow functions does not create it's own execution context but inherits this from outside function where the function is defined.
Therefore, the problem in the above code can be fixed just br replacing the regular function for the anonymous function with arrow function.
function cookFood(dish) {
this.dish = dish;
this.time = time;
function setTime(sec) {
setTimeout(() => {
console.log(this.dish + " cooked for " + sec + " seconds");
}, sec * 1000);
}
}
let curry = new cookFood("Curry"); // this.dish = "Curry"
curry.setTime(3); // Output: Curry cooked for 2 secondsSince, arrow functions do not create their own this context. the this will refer to the cookFood object we created.
Function borrowing is a concept that allows us to reuse the methods of one object on another object. Simply put, we can borrow the methods of one object and use them on other object.
Implicit binding is the default way that this is determined in a function call. Whenever we invoke a method of an object using the dot notation, the this keyword will point to the object with which it was invoked. In simple terms, when a function is called as a method of an object, this is set to the object itself.
const user = {
name: "John",
greetUser: function () {
console.log("Welcome to JavaScript " + this.name);
},
};
user.greetUser(); // Output: "Welcome to JavaScript John"In this example, greetUser() is called as a method of user, so this is set to user.
Explicit binding in JavaScript refers to the technique of manually setting the this keyword inisde a function using the call(), apply() or bind() methods. We use explicit binding when we want to access the methods of an object with the values provided by another object.
The bind() method creates and returns a new function, whose this keyword is set to object which was passed to it. It binds a method to an object and returns to be invoked later.
function greet() {
console.log("Welcome " + this.name);
}
const user1 = {
name: "Prabesh",
};
const user2 = {
name: "Laxmi",
};
const greetUser1 = greet.bind(user1);
const greetUser2 = greet.bind(user2);
greetUser1(); // Output: "Welcome Prabesh"
greetUser2(); // Output: "Welcome Laxmi"In the above example, we have function greet() and objects user1 and user2 with name properties. We are using the bind() method to create two new functions greetUser1() and greetUser2() which
have their this value bound to user1 and user2 respectively.
Let's have a look at another example:
const user = {
firstName: "Super",
lastName: "Mario",
greet: function () {
console.log("Hello, I am " + this.firstName + " " + this.lastName + ".");
},
};
user.greet(); // Output: "Hello, I am Super Mario."
const user2 = {
firstName: "Bob",
lastName: "Builder",
};
const intro = user.greet.bind(user2);
intro(); // Output: Hello, I am Bob Builder.Here, we defined an object call user that has three properties: firstName, lastName and greet. The greet property is a function that logs a message including firstName and lastName. When we call user.greet(), we are invoking the greet function with this referring to user object. Hence, we get a output of "Hello, I am Super Mario". This concept is referred as impilicit binding.
Next, we define another object called user2 that has two properties: firstName and lastName. We then create a function called intro by using the bind() method to bind the this keyword of the greet function to the user2 object. This means when we call intro() function, this inside the greet function will refer to the user2 object and the output will be "Hello, I am Bob Builder."
To summarize, by using bind(), we are essentially creating a new function that has its this value set to user2 object. This allows us to resue the greet function with a different object. This concept is referred as explicit binding.
The call() method works similar to bind() but has one major difference. As we know, bind() method returns a function that can be invoked later but the call() method invokes a function and returns the result.
function greet() {
console.log("Hello, my name is " + this.name);
}
const person = {
name: "Ninja",
};
greet.call(person); // Output: "Hello, my name is Ninja"Here, we hava a greet function and an object person with a name property. We are using the call() method to call greet() function with person as the context to this. As a result, we get an output of "Hello, my name is Ninja".
let us consider another example:
const user = {
firstName: "Sonic",
lastName: "Hedgehog",
greet: function (place) {
console.log(`I am ${this.firstName} ${this.lastName} from ${place}`);
},
};
user.greet("Colorado"); // Output: I am Sonic Hedgehog from Colorado
const user2 = {
firstName: "Ninja",
lastName: "Turtle",
};
user.greet.call(user2, "Texas"); // Output: I am Ninja Turtle from TexasIn this example, we have two objects: user and user2. We are borrowing the greet() function from user object using call() method abnd binding it with user2 object.
By using the call() method, we are invoking the greet function in the context of user2 object. The first argument to call() is the object that this should refer to inside the fucntion, and any subsequent arguments are passed as arguments to the function itself.
The apply() method is similar to call() method, but takes an array of arguments instead of inidividual arguments. In the call() method, we can pass multiple parameters, but in apply() method, we have to pass in single array that would contain all the parameters.
const user = {
firstName: "Cristiano",
lastName: "Ronaldo",
greet: function (club, country) {
console.log(
`I am ${this.firstName} ${this.lastName} from ${country} playing for ${club}.`
);
},
};
user.greet("Al-Nassr", "Portugal"); // Output: I am Cristiano Ronaldo from Portugal playing for Al-Nassr.
const user2 = {
firstName: "Lionel",
lastName: "Messi",
};
user.greet.apply(user2, ["PSG", "Argentina"]); // Output: I am Lionel Messi from Argentina playing for PSG.By using apply(), we're essentially invoking the greet function in the context of the user2 object. The first argument to apply() is the object that this should refer to inside the function, and the second argument is an array of arguments to pass to the function itself. In this case, we're passing an array of two elements, "PSG" and "Argentina", which are used as the club and country parameters in the greet function, respectively.
Summary:
- By default, the methods of an object are implicitly bounded to the object itself, and we can access them using the dot (.) operator.
- To access the methods of other objects, we need to explicitly bind them to the object using the
bind(),call(), orapply()methods, with each one of them having its own use cases.