JavaScript Methods and this Keyword

Before continuing this tutorial, make sure you are familiar with:

JavaScript Method

A JavaScript method is a function defined within an object. For example,

// dog object
const dog = {
    name: "Rocky",

// bark method bark: function () { console.log("Woof!"); }
// access method dog.bark();
// Output: Woof!

In the above example, the dog object has two keys: name and bark.

Since the bark key holds a function, we refer to it as a method.

Notice that we accessed the bark() method using dog.bark(). Thus, the syntax to access an object method is:


JavaScript this Keyword

We use this keyword in an object method to access a property of the same object. For example,

// person object
const person = {
    name: "John",
    age: 30,

    // method
    introduce: function () {
console.log(`My name is ${} and I'm ${this.age} years old.`);
} }; // access the introduce() method person.introduce(); // Output: My name is John and I'm 30 years old.

In the above example, we created the person object with two properties (name and age) and a method introduce().

Inside the introduce() method, we used and this.age to refer to the name and age keys of the person object.

To learn more, visit JavaScript this.

Add Methods to an Object

You can add more methods to a JavaScript object even after we've defined it. For example,

// student object
let student = {
    name: "John"

// add new method student.greet = function () { console.log("Hello"); };
// access greet() method student.greet(); // Output: Hello

In the above example, we created the student object with the property name: "John".

Initially, student did not have any method. So, we used the dot notation to add a new method to the object:

student.greet = function() {

JavaScript Built-In Methods

JavaScript provides a wide array of useful methods known as built-in methods. Some commonly used built-in methods (and the respective objects they belong to) are given in the table below:

Method Object Description
console.log() Console Displays messages or variables in the browser's console.
prompt() Window Displays a dialog box that prompts the user for input.
concat() String Concatenates the arguments to the calling string.
toFixed() Number Rounds off a number into a fixed number of digits.
sort() Array Sorts the elements of an array in specific order.
random() Math Returns a pseudo-random float number between 0 and 1.

To learn more about JavaScript built-in methods, visit JavaScript Built-In Methods.

Examples: JavaScript Build-In Methods

JavaScript concat() Method

We use the concat() method to concatenate (join) two strings. For example,

let firstName = "Tony ";
let lastName = "Stark";

// built-in string method concat() // join lastName to the end of firstName let fullName = firstName.concat(lastName);
console.log(fullName); // Output: Tony Stark

Here, we used concat() to concatenate firstName and lastName.

JavaScript toFixed() Method

We use the toFixed() method to round off a number into a fixed number of digits. For example,

let num = 5.12345;

// built-in number method toFixed() // round off num to two decimal places let roundedNum = num.toFixed(2);
console.log(roundedNum); // Output: 5.12

Here, we use toFixed(2) to round off the value of num from 5.12345 to 5.12.

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