JavaScript Basics

JavaScript Data Types

JavaScript data types define what kind of value a variable can store.

A variable can store text, numbers, true or false values, lists, objects, and more.

Example

javascript

let name = "Amit";
let age = 25;
let isStudent = true;

In the above example:

Variable	Value	Data Type
`name`	`"Amit"`	String
`age`	`25`	Number
`isStudent`	`true`	Boolean

Each variable stores a different type of value, and JavaScript treats those values differently.

Why Data Types Matter

Data types are important because JavaScript handles different values in different ways.

For example, JavaScript adds numbers, but it joins strings together.

Example with numbers:

Example

javascript

let a = 10;
let b = 20;

console.log(a + b);

Here, JavaScript adds the numbers and gives the result 30.

Example with strings:

Example

javascript

let a = "10";
let b = "20";

console.log(a + b);

Here, JavaScript joins the values and gives the result 1020.

So, understanding data types helps you avoid wrong results in your code.

Main JavaScript Data Types

JavaScript has several data types.

Data Type	Description	Example
String	Text value	`"Hello"`
Number	Numeric value	`25`
Boolean	True or false value	`true`
Undefined	Variable declared without value	`undefined`
Null	Empty value	`null`
Object	Collection of related data	`{ name: "Amit", age: 25 }`
Array	List of values	`["HTML", "CSS", "JavaScript"]`
BigInt	Very large integer	`123456789n`
Symbol	Unique value	`Symbol("id")`

For beginners, String, Number, Boolean, Undefined, Null, Object, and Array are the most important.

String

A string is used to store text.

Strings are written inside quotes.

Example

javascript

let firstName = "Amit";
let course = "JavaScript";

You can use single quotes or double quotes.

Example

javascript

let city = 'Delhi';
let country = "India";

Both are valid.

Number

A number is used to store numeric values.

Example

javascript

let age = 25;
let price = 499.99;

JavaScript uses the same Number type for both whole numbers and decimal numbers.

Example

javascript

let a = 10;
let b = 5;

let total = a + b;

Boolean

A boolean has only two values:

Syntax

javascript

true
false

Booleans are mostly used in conditions.

Example

javascript

let isLoggedIn = true;
let isAdmin = false;

Boolean values are useful when you want to check whether something is yes or no, active or inactive, correct or incorrect.

Undefined

A variable is undefined when it is declared but no value is assigned.

Example

javascript

let message;

console.log(message);

Here, message exists, but it has no value yet.

Null

null means empty or no value.

It is used when you intentionally want to keep a variable empty.

Example

javascript

let selectedUser = null;

Here, selectedUser has no value right now.

Difference between undefined and null:

Value	Meaning
`undefined`	Value is not assigned
`null`	Empty value is assigned intentionally

Object

An object is used to store related data in one variable.

Example

javascript

let student = {
  name: "Amit",
  age: 20,
  course: "JavaScript"
};

You can access object values using property names.

Example

javascript

console.log(student.name);
console.log(student.course);

Objects are useful when you want to store information about one thing, such as a student, user, product, or book.

Array

An array is used to store multiple values in a single variable.

Example

javascript

let languages = ["HTML", "CSS", "JavaScript"];

Array values are accessed using index numbers.

Example

javascript

console.log(languages[0]);
console.log(languages[1]);
console.log(languages[2]);

Array index starts from 0.

Index	Value
`0`	HTML
`1`	CSS
`2`	JavaScript

BigInt

BigInt is used for very large integer numbers.

Example

javascript

let bigNumber = 12345678901234567890n;

The n at the end makes it a BigInt value.

For most beginner-level programs, normal numbers are enough.

Symbol

A Symbol is a unique value.

Example

javascript

let id = Symbol("id");

Symbol is mostly used in advanced JavaScript, so beginners do not need to use it much in the beginning.

typeof Operator

The typeof operator is used to check the data type of a value.

Example

javascript

let name = "Amit";
let age = 25;
let isStudent = true;

console.log(typeof name);
console.log(typeof age);
console.log(typeof isStudent);

The typeof operator is very useful when you want to check what type of value a variable contains.

Complete Example

html

<!DOCTYPE html>
<html>
<head>
  <title>JavaScript Data Types</title>
</head>
<body>

  <h1>JavaScript Data Types</h1>

  <p id="result"></p>

  <script>
    let name = "Amit";
    let age = 25;
    let isStudent = true;
    let course;
    let selectedBook = null;
    let skills = ["HTML", "CSS", "JavaScript"];

    let student = {
      name: "Rahul",
      age: 20
    };

    document.getElementById("result").innerHTML =
      "Name: " + name + "<br>" +
      "Age: " + age + "<br>" +
      "Student: " + isStudent + "<br>" +
      "Course: " + course + "<br>" +
      "Selected Book: " + selectedBook + "<br>" +
      "Skill: " + skills[2] + "<br>" +
      "Object Name: " + student.name;
  </script>

</body>
</html>

Try It Yourself

Run the above example in the Try It Editor.

Try changing the values of name, age, isStudent, skills, and student.

Also try using the typeof operator with different variables.

Important Notes

JavaScript variables can store different types of values.
Strings are written inside quotes.
Numbers are written without quotes.
Boolean values are either true or false.
A variable without a value is undefined.
null is used when you intentionally want to store an empty value.
Objects store related data in key-value pairs.
Arrays store multiple values in a list.
The typeof operator is used to check data types.

Conclusion

JavaScript data types define the type of value stored in a variable.

The main JavaScript data types are String, Number, Boolean, Undefined, Null, Object, Array, BigInt, and Symbol.

Understanding data types is important because it helps you write correct JavaScript code and avoid unexpected results.

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