JavaScript · 2 min read

Variables and Data Types

Store values with let and const, and learn the core data types: strings, numbers, booleans, null, and undefined.

A variable is a named container for a value. You create variables with let or const.

let score = 0;          // can be reassigned later
const name = "Ada";     // cannot be reassigned
 
score = 10;             // ✅ fine
// name = "Grace";      // ❌ TypeError: Assignment to constant variable

Naming rules

Names can contain letters, digits, _, and $, but cannot start with a digit.
They are case-sensitive: age and Age are different.
Use camelCase for multi-word names: totalPrice, isLoggedIn.
Choose descriptive names: userAge beats x.

Data types

JavaScript has a handful of primitive types:

Type	Example	Notes
String	`"hello"`, `'hi'`	Text
Number	`42`, `3.14`, `-7`	Integers and decimals share one type
Boolean	`true`, `false`	Yes or no
Undefined	`undefined`	A variable declared but not given a value
Null	`null`	Intentionally "no value"
BigInt	`9007199254740993n`	Very large integers
Symbol	`Symbol("id")`	Unique identifiers (advanced)

Everything else (arrays, functions, dates) is an object, covered later.

Check a type with typeof:

typeof "hi";      // "string"
typeof 42;        // "number"
typeof true;      // "boolean"
typeof undefined; // "undefined"
typeof null;      // "object"  (a famous historical bug!)

Strings

const first = "Ada";
const last = 'Lovelace';
 
// Joining strings (concatenation)
const full = first + " " + last;   // "Ada Lovelace"
 
// Template literals use backticks and can embed expressions
const intro = `Hi, I'm ${first}. 2 + 2 is ${2 + 2}.`;
 
// Useful properties and methods
full.length;            // 12
full.toUpperCase();     // "ADA LOVELACE"
full.includes("Love");  // true
full.slice(0, 3);       // "Ada"
"  padded  ".trim();    // "padded"

Numbers

const price = 19.99;
const qty = 3;
const total = price * qty;       // 59.97
 
Math.round(4.6);    // 5
Math.floor(4.6);    // 4
Math.max(3, 9, 1);  // 9
Math.random();      // a random number from 0 up to (not including) 1
 
Number("42");       // 42 (string to number)
(59.971).toFixed(2); // "59.97" (number to string with 2 decimals)

Try it

<p id="out"></p>

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Summary

Declare variables with const (preferred) or let.
Primitive types: string, number, boolean, undefined, null, bigint, symbol.
Use typeof to check a value's type.
Template literals (backticks) make building strings easy.

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Introduction to JavaScript Operators and Conditions