• +91-9142290450

  • Hajipur, Bihar, 844101

☰
❮
❯

JavaScript

coding learning websites codepractice

JS Basics

JS Variables & Operators

JS Data Types & Conversion

JS Numbers & Math

JS Strings

JS Dates

JS Arrays

JS Control Flow

JS Loops & Iteration

JS Functions

JS Objects

JS Classes & Modules

JS Async Programming

JS Advanced

JS HTML DOM

JS BOM (Browser Object Model)

JS Web APIs

JS AJAX

JS JSON

JS Graphics & Charts

JS Data Types

← Previous Next →

JavaScript data types define the kind of values that a variable can store and how those values behave during operations. Understanding data types is one of the most important concepts in JavaScript because every program works with data in some form. Whether you are building a simple calculator, a form validation system, or a full-scale web application, knowing how JavaScript handles different types of data helps you write reliable and predictable code.

In this chapter, you will learn what data types are, why they matter, the different types available in JavaScript, how values behave with each type, practical examples, common pitfalls, best practices, and real-world usage scenarios.

What Are Data Types

A data type tells JavaScript what kind of value a variable holds. For example, a variable may store a number, a text value, a boolean result, or a collection of values. JavaScript is a dynamically typed language, which means you do not need to specify the data type when declaring a variable. The type is automatically determined based on the value assigned to it.

let age = 22;
let name = "Ananya";
let isStudent = true;

In this example, JavaScript automatically understands that age is a number, name is a string, and isStudent is a boolean.

Why Data Types Are Important

Data types help JavaScript decide how to handle values during operations. Adding two numbers produces a numeric result, while adding two strings joins them together. Using the wrong type can lead to unexpected results or bugs.

Understanding data types allows you to:

  • Perform correct calculations

  • Validate user input properly

  • Avoid runtime errors

  • Write cleaner and more predictable code

  • Debug issues more efficiently

Categories of JavaScript Data Types

JavaScript data types are broadly divided into two categories:

  • Primitive data types

  • Non-primitive or reference data types

Each category serves a different purpose and behaves differently in memory.

Primitive Data Types

Primitive data types store a single simple value. They are immutable, meaning their values cannot be changed directly. JavaScript has seven primitive data types.

Number

The number data type is used to store numeric values, including integers and floating-point numbers. JavaScript does not have separate types for integers and decimals.

let ageAnanya = 22;
let heightMira = 5.4;
let temperature = -10;

Numbers can be used in arithmetic operations like addition, subtraction, multiplication, and division.

let totalMarks = 450;
let subjects = 5;
let averageMarks = totalMarks / subjects;

Special numeric values also exist, such as Infinity, -Infinity, and NaN (Not a Number).

String

The string data type is used to store text. Strings are enclosed in single quotes, double quotes, or backticks.

let firstName = "Ishita";
let lastName = 'Sharma';
let fullName = firstName + " " + lastName;

Strings can include letters, numbers, spaces, and symbols. JavaScript provides many built-in methods to work with strings, such as finding length, converting case, or extracting parts of a string.

Boolean

The boolean data type stores one of two values: true or false. Booleans are commonly used in conditions and decision-making.

let isLoggedIn = true;
let hasPermission = false;

Booleans are often the result of comparisons.

let ageRiya = 18;
let isAdult = ageRiya >= 18;

Undefined

The undefined data type means a variable has been declared but not assigned a value.

let score;
console.log(score); // undefined

This often happens when a value is expected but has not been provided yet.

Null

The null data type represents an intentional absence of value. It is assigned explicitly by the programmer.

let selectedCourse = null;

Although null looks similar to undefined, it is used deliberately to indicate that a variable has no value.

BigInt

The BigInt data type is used to store very large numbers that exceed the safe limit of the number type.

let bigNumber = 123456789012345678901234567890n;

BigInt values are useful in applications dealing with large integers such as financial systems or cryptography.

Symbol

The symbol data type is used to create unique identifiers. Each symbol is unique, even if they have the same description.

let id1 = Symbol("id");
let id2 = Symbol("id");

Symbols are commonly used in advanced JavaScript scenarios such as object property keys that should not conflict with others.

Non-Primitive Data Types

Non-primitive data types can store multiple values or complex structures. These types are mutable, meaning their contents can be changed.

Object

The object data type is used to store data in key-value pairs.

let student = {
    name: "Sanya",
    age: 21,
    course: "JavaScript"
};

Objects are widely used to represent real-world entities and structured data.

Array

Arrays are a special type of object used to store ordered lists of values.

let students = ["Ananya", "Ishita", "Mira", "Riya"];

Arrays allow you to store multiple values in a single variable and access them using indexes.

Function

Functions are reusable blocks of code and are also treated as objects in JavaScript.

function greet(name) {
    return "Hello " + name;
}

Functions can be assigned to variables, passed as arguments, and returned from other functions.

How Data Types Behave in Memory

Primitive data types are stored by value. When you assign a primitive value to another variable, a copy is created.

let age1 = 20;
let age2 = age1;
age2 = 25;

Changing age2 does not affect age1.

Non-primitive data types are stored by reference. When you assign an object or array to another variable, both variables point to the same memory location.

let student1 = { name: "Diya", age: 22 };
let student2 = student1;
student2.age = 25;

Here, changing student2 also changes student1.

Common Mistakes

  • Confusing null and undefined

  • Expecting JavaScript to throw errors for type mismatches

  • Forgetting that arrays and objects are reference types

  • Assuming all numbers are safe without checking limits

Best Practices

  • Use meaningful variable names to indicate expected data type

  • Initialize variables whenever possible

  • Be careful when copying objects and arrays

  • Validate user input before processing

  • Understand type behavior during operations

Real-World Applications

JavaScript data types are used everywhere in web development:

  • Numbers for calculations, prices, and scores

  • Strings for names, messages, and form input

  • Booleans for conditions and permissions

  • Arrays for lists of items or users

  • Objects for structured data like profiles and settings

Summary of JS Data Types

JavaScript data types define how values are stored and manipulated. Primitive types handle simple values, while non-primitive types manage complex data structures. Understanding how each data type works, how they behave in memory, and when to use them is essential for writing effective and bug-free JavaScript code.

Practice Questions

Q1. How do you declare a string variable named greeting with the value "Hello World"?

Q2. How do you declare a number variable price with the value 199.99?

Q3. How do you check if a variable isActive is a boolean using the typeof operator?

Q4. How do you define an array fruits containing "apple", "banana", and "mango"?

Q5. How do you create an object person with properties name: "John" and age: 30?

Q6. How do you define a variable discount with the value null and check its type using typeof?

Q7. How do you define a function greet() that prints "Hi" and check its type?

Q8. How do you assign a BigInt value 12345678901234567890n to a variable bigNum?

Q9. How do you declare an undefined variable named result?

Q10. How do you create a variable now and assign the current date and time using Date() object?

← Previous Next →

Try a Short Quiz.

coding learning websites codepractice

No quizzes available.

JavaScript

online coding class codepractice

JS Basics

JS Variables & Operators

JS Data Types & Conversion

JS Numbers & Math

JS Strings

JS Dates

JS Arrays

JS Control Flow

JS Loops & Iteration

JS Functions

JS Objects

JS Classes & Modules

JS Async Programming

JS Advanced

JS HTML DOM

JS BOM (Browser Object Model)

JS Web APIs

JS AJAX

JS JSON

JS Graphics & Charts

codepractice logo
Login
Go Back Top