What are Symbols and where are they used?

Question

Accepted Answer

A **Symbol** is a unique, immutable primitive. Every `Symbol()` you create is **guaranteed distinct** — even two with the same description are not equal. Their main job is to be **collision-proof object keys**.

```js
const id = Symbol("id");
const id2 = Symbol("id");
id === id2;        // false — always unique

const user = { [id]: 123, name: "Ann" };
user[id];          // 123
```

## Why unique keys are useful

Symbol-keyed properties don't collide with string keys and are **hidden** from normal enumeration — perfect for attaching metadata without polluting the object:

```js
Object.keys(user);          // ["name"]  — symbol key is skipped
for (const k in user) {}    // skips the symbol
JSON.stringify(user);       // {"name":"Ann"} — symbol not serialized
Object.getOwnPropertySymbols(user); // [Symbol(id)] — only way to list them
```

So a library can store internal data on your object via a Symbol without risking a clash with your properties or showing up in your loops.

## Well-known symbols customize language behavior

The most important use — `Symbol.iterator` makes an object iterable:

```js
const range = {
  [Symbol.iterator]() {
    let i = 1;
    return { next: () => (i <= 3 ? { value: i++, done: false } : { done: true }) };
  },
};
[...range]; // [1, 2, 3]
```

Others: `Symbol.asyncIterator`, `Symbol.toPrimitive` (custom coercion), `Symbol.hasInstance`.

## Global registry

```js
Symbol.for("key"); // shared symbol across the whole app (and across realms)
```

## Why it matters

Symbols enable non-colliding metadata keys and let you hook into core language protocols (iteration, coercion) — they're how built-ins and frameworks extend objects safely.