Variables and Data Types

What is a variable?

A variable is a labelled box that holds a value. You put something in the box, give the box a name, and later you can look inside using that name.

let userName = 'Aditi';
console.log(userName);

Here userName is the label and 'Aditi' is the value inside.

Three ways to declare a variable

JavaScript has three keywords for creating variables. In modern code you only need two of them.

let for values that change

let score = 0;
score = 10;
score = 25;
console.log(score);

const for values that stay the same

const birthYear = 1998;

If you try to reassign a const, JavaScript stops you:

const birthYear = 1998;
birthYear = 2000;

That last line throws TypeError: Assignment to constant variable.

var is the old way

var was the only option before 2015. It behaves in surprising ways that cause bugs, so treat it as something you read in old code but never write yourself.

Rule of thumb: Start with const. Switch to let only when you know the value has to change. Never use var.

Naming rules

A variable name:

• can contain letters, digits, _ and $
• cannot start with a digit
• cannot be a reserved word like class or return
• is case sensitive, so age and Age are two different variables

The common style in JavaScript is camelCase, where the first word is lowercase and every word after it starts with a capital:

let firstName = 'Ravi';
let totalItemCount = 12;
let isLoggedIn = true;

Pick names that describe the value. n tells the reader nothing, numberOfStudents tells them everything.

The data types

Every value in JavaScript has a type. There are seven primitive types plus objects.

String

Text, wrapped in quotes. Single, double, or backticks all work.

const city = 'Kolkata';
const greeting = "Hello there";
const message = `Welcome to ${city}`;
console.log(message);

Number

JavaScript has one number type for whole numbers and decimals alike.

const age = 27;
const price = 149.99;
const negative = -5;
console.log(age + price);

Boolean

Only two possible values: true or false. These drive every decision your program makes.

const isRaining = false;
const hasAccount = true;

Undefined

A variable that was declared but never given a value.

let favouriteColour;
console.log(favouriteColour);

That prints undefined.

Null

An empty value that you set on purpose. undefined means “nobody filled this in yet”, while null means “this is deliberately empty”.

let selectedItem = null;

BigInt and Symbol

Two specialist types you will rarely need early on. BigInt holds whole numbers larger than Number can handle, and Symbol creates unique identifiers.

const huge = 9007199254740993n;
const id = Symbol('id');

Object

Everything that is not a primitive is an object. Arrays, functions, and dates are all objects.

const person = { name: 'Meera', age: 31 };
const colours = ['red', 'green', 'blue'];

Checking a type

Use typeof when you want to know what you are holding.

console.log(typeof 'hello');
console.log(typeof 42);
console.log(typeof true);
console.log(typeof undefined);
console.log(typeof { a: 1 });

One famous oddity: typeof null returns 'object'. This is a bug from the very first version of JavaScript that was never fixed because too much code depends on it. Just remember it and move on.

Primitives are copied, objects are shared

This trips up almost every beginner, so it is worth seeing early.

let a = 10;
let b = a;
b = 20;
console.log(a);

a is still 10, because the number was copied into b.

const list1 = ['x'];
const list2 = list1;
list2.push('y');
console.log(list1);

Now list1 is ['x', 'y']. Both names point at the same array, so changing one changes the other.

Quick recap

• const by default, let when the value must change, var never.
• Names use camelCase and should describe what they hold.
• Primitive types: string, number, boolean, undefined, null, BigInt, symbol.
• Everything else is an object.
• typeof tells you the type, and typeof null lying about being an object is a known quirk.