JavaScript Data Types

JavaScript Data Types

Every value in JavaScript has a type. Understanding JavaScript's data types is essential because the type of a value determines what operations you can perform on it and how it behaves in expressions. JavaScript is a dynamically typed language, meaning variables can hold any type without being declared as a specific type.

Primitive Types

JavaScript has seven primitive types. A primitive is an immutable value — when you manipulate it, you get a new value rather than modifying the original.

A string is a sequence of characters enclosed in single quotes, double quotes, or backticks. Use backticks for template literals that embed expressions.

const firstName = 'Alice';
const greeting = `Hello, ${firstName}!`;
console.log(greeting); // 'Hello, Alice!'

A number in JavaScript represents all numeric values, including integers and decimals. There is no separate integer type.

const age = 30;
const price = 9.99;
const negative = -42;
console.log(Number.isInteger(age)); // true

A boolean is either true or false. Booleans are produced by comparisons and used in conditional logic.

null is an intentional absence of value. You assign it explicitly to indicate that a variable intentionally holds no value. undefined means a variable has been declared but not yet assigned a value.

let selectedItem = null; // no item chosen yet
let inputValue; // undefined — declared but not assigned
console.log(typeof null);      // 'object' (historic quirk)
console.log(typeof undefined); // 'undefined'

The typeof Operator

Use typeof to check a value's type at runtime. This is especially useful when handling dynamic data.

console.log(typeof 'hello');    // 'string'
console.log(typeof 42);         // 'number'
console.log(typeof true);       // 'boolean'
console.log(typeof undefined);  // 'undefined'
console.log(typeof {});         // 'object'
console.log(typeof []);         // 'object' (arrays are objects)
console.log(typeof function(){}); // 'function'

Type Coercion

JavaScript automatically converts types in certain situations, a process called implicit coercion. This can produce surprising results. Always use === (strict equality) instead of == (loose equality) to avoid coercion issues.

console.log('5' + 3);   // '53' — number coerced to string
console.log('5' - 3);   // 2  — string coerced to number
console.log('' == false); // true  — loose equality with coercion
console.log('' === false); // false — strict equality, no coercion