# Symbols

`Symbol` is a primitive data type introduced in ES6. A Symbol is a unique and immutable identifier.

## 1. Creating Symbols

You create a symbol using the `Symbol()` factory function. You can pass an optional description string.

```javascript
const sym1 = Symbol();
const sym2 = Symbol("foo");
const sym3 = Symbol("foo");

console.log(sym2 === sym3); // false
```

Every `Symbol()` call is guaranteed to return a unique Symbol. Even if they have the same description, they are different values.

## 2. Use Cases

### Unique Object Keys
Symbols are often used as property keys in objects to avoid name collisions.

```javascript
const id = Symbol("id");

const user = {
  name: "John",
  [id]: 123
};

console.log(user[id]); // 123
```

### "Hidden" Properties
Symbol properties do not appear in `for...in` loops or `Object.keys()`.

```javascript
for (let key in user) {
  console.log(key); // "name" (id is skipped)
}

console.log(Object.keys(user)); // ["name"]
```

However, they are not truly private. You can access them using `Object.getOwnPropertySymbols(user)`.

## 3. Global Symbol Registry

Usually, symbols are unique. However, sometimes you want to share a symbol across different parts of your application (or even across realms like iframes).

*   `Symbol.for(key)`: Searches for a symbol with the given key in the global registry. If found, returns it. If not, creates a new one and stores it.
*   `Symbol.keyFor(sym)`: Returns the key for a global symbol.

## 4. Well-Known Symbols

JavaScript uses several built-in symbols to define language-internal behavior. You can use these to customize how your objects behave.

*   `Symbol.iterator`: A method that returns the default iterator for an object. Used by `for...of`.
*   `Symbol.toPrimitive`: A method that converts an object to a primitive value.
*   `Symbol.toStringTag`: A string value used for the default description of an object.

[[programming/javascript/vanilla/javascript]]
[[programming/javascript/vanilla/data-types]]
[[programming/javascript/vanilla/iterators-and-iterables]]