Mit tesz az `as const` assertion?

Question

Accepted Answer

`as const` is a **const assertion** that tells TypeScript to infer the **most specific (literal), deeply readonly** type for a value, instead of widening it.

```ts
// without as const — widened, mutable
let a = "up";            // type: string
const obj = { x: 1 };     // type: { x: number }
const arr = [1, 2];       // type: number[]

// with as const — narrow literals, readonly
const b = "up" as const;          // type: "up"
const obj2 = { x: 1 } as const;    // type: { readonly x: 1 }
const arr2 = [1, 2] as const;      // type: readonly [1, 2] (a tuple!)
```

It does three things: widens *nothing* (keeps literal values), makes everything `readonly`, and turns arrays into **tuples**.

## Miért olyan hasznos

**Deriving a union type from a values list (single source of truth):**

```ts
const ROLES = ["admin", "editor", "viewer"] as const;
type Role = typeof ROLES[number]; // "admin" | "editor" | "viewer"
// add a role to the array → the union updates automatically
```

**Preserving literal types for config/state:**

```ts
const action = { type: "INCREMENT", by: 1 } as const;
// action.type is "INCREMENT" (a literal), so it works in discriminated unions
// without as const it'd be `string`, breaking narrowing
```

## Miért fontos

`as const` bridges runtime values and precise types.

It lets you define data once (an array/object) and derive exact literal types from it — invaluable for action types, route maps, config, and any discriminated-union tag that must stay a literal rather than widening to `string`.