Primitive and Reference Types

JavaScript primitive types are string, number, bigint, boolean, undefined, symbol, and null (although typeof null has a historical quirk). Objects include ordinary objects, arrays, and functions.

const title = "JavaScript"; // string
const rating = 4.8;         // number
const isPublished = true;   // boolean
let missingValue;           // undefined
const nothing = null;       // intentional absence
const largeId = 9007199254740993n; // bigint literal
const uniqueKey = Symbol("key");

JavaScript's ordinary number type uses IEEE 754 floating-point numbers. For integer arithmetic, the largest exactly representable integer is Number.MAX_SAFE_INTEGER. BigInt supports larger integers but cannot be mixed directly with number in arithmetic.

Primitive values versus objects#

Primitive values are copied as values. Variables referencing objects hold a reference to the object, so two variables can refer to the same object:

const original = { count: 1 };
const alias = original;
alias.count = 2;
console.log(original.count); // 2

Assignment does not automatically clone an object. A shallow copy can be made with { ...original }, but nested objects remain shared unless copied separately.

typeof#

typeof "hello" returns "string"; typeof 42 returns "number"; and typeof undefined returns "undefined". Arrays and null return "object", so use Array.isArray(value) to check for an array and explicit comparisons to check for null.

Practice#

Create two variables that reference one object, mutate it through the second variable, and explain why reading the first variable shows the changed value.