Data Types

JavaScript's Eight Data Types

JavaScript is dynamically typed: variables do not have a fixed type, values do. There are seven primitive types and one object type.

• number — integers and floats (64-bit IEEE 754)
• string — text, always immutable
• boolean — true or false
• undefined — a variable that has been declared but not assigned
• null — an intentional absence of a value
• bigint — arbitrary-precision integers, e.g. 9007199254740993n
• symbol — guaranteed-unique identifiers
• object — everything else: plain objects, arrays, functions, dates, …

console.log(typeof 42);          // "number"
console.log(typeof "hello");     // "string"
console.log(typeof true);        // "boolean"
console.log(typeof undefined);   // "undefined"
console.log(typeof null);        // "object" (historical bug!)
console.log(typeof {});          // "object"
console.log(typeof []);          // "object"
console.log(typeof function(){}); // "function"

Primitives vs Objects

Primitives are immutable and compared by value. Objects are mutable and compared by reference — two objects with the same content are not equal unless they are the same object in memory.

const a = { x: 1 };
const b = { x: 1 };
console.log(a === b); // false — different references

const c = a;
console.log(a === c); // true — same reference