What are Symbols and what are their main use cases?

A Symbol is a primitive that is guaranteed globally unique. Used mainly as object property keys to avoid name collisions.

const id = Symbol('id');
const id2 = Symbol('id');
id === id2; // false — always unique even with same description

const user = {};
user[id] = 42; // Symbol as property key

// Symbols are invisible to normal enumeration
Object.keys(user);                          // []
JSON.stringify(user);                       // '{}' — symbols excluded
Object.getOwnPropertySymbols(user);         // [Symbol(id)] — explicit access

// Well-known Symbols — customize built-in behavior
class MyIterable {
  [Symbol.iterator]() {      // makes instances work in for...of
    let n = 0;
    return { next: () => n < 3
      ? { value: n++, done: false }
      : { done: true } };
  }
}

for (const v of new MyIterable()) console.log(v); // 0, 1, 2

// Other well-known Symbols:
// Symbol.toPrimitive — control type coercion
// Symbol.hasInstance — customize instanceof
// Symbol.toStringTag — customize Object.prototype.toString output