JavaScript Primitive Values

Every value in JavaScript has a type. There are two groups: primitive values (simple, single values) and objects (everything else, such as arrays and functions).

The seven primitives

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const name = "Maya";        // string
const age = 28;             // number
const isStudent = false;    // boolean
let nothingYet;             // undefined
const empty = null;         // null

console.log(name, age, isStudent, nothingYet, empty);

Primitives are immutable

You cannot change a primitive value. You can only replace it with another.

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let word = "cat";
word[0] = "b";       // silently ignored
console.log(word);   // → cat

word = "bat";        // replacing the whole value works
console.log(word);   // → bat

String methods never change the original. They return a new string.

Primitives vs objects

Primitives are compared and copied by value. Objects are handled by reference, which means two variables can point at the same object.

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let a = 5;
let b = a;
b = 10;
console.log(a, b); // → 5 10

const first = { n: 1 };
const second = first;   // both names point at the same object
second.n = 99;
console.log(first.n);   // → 99

Changing second changed first, because they are the same object. You will see this again in the Objects chapter.

The next lessons look at each type in turn: undefined and null, booleans, numbers, strings, BigInt and Symbol.