Symbol

Symbol is a primitive type introduced in ES6. Every symbol is guaranteed to be unique. They are often used as keys for object properties to avoid name collisions.

let sym1 = Symbol();
let sym2 = Symbol("id"); // optional description
let sym3 = Symbol("id");

console.log(sym2 === sym3); // false – each Symbol is unique

Using as property keys:

const obj = {};
const id = Symbol("id");
obj[id] = "secret value";
console.log(obj[id]); // 'secret value'
console.log(Object.keys(obj)); // [] – symbols are not enumerated

Well‑known symbols like Symbol.iterator are used to define built‑in behaviors.

const arr = [1, 2, 3];
const iterator = arr[Symbol.iterator](); // returns an iterator

Symbols are not fully private but offer a way to create non‑string property keys that won’t clash with other keys.