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:
objectName.methodKey()
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 ${this.name} 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 this.name 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() {
console.log("Hello");
};
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
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.
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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