`as const` 断言有什么作用?

Question

`as const` 断言有什么作用?

Accepted Answer

`as const` 是一个 **const 断言**，它告诉 TypeScript 为一个值推断 **最具体（字面量）、深度只读** 的类型，而不是将其扩展。

```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!)
```

它做三件事：不进行任何扩展（保持字面量值）、使所有内容都变为 `readonly`，以及将数组转换为 **元组**。

## 为什么这很重要

**从值列表派生联合类型（单一事实来源）：**

```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
```

**为配置/状态保留字面量类型：**

```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
```

## 为什么这很重要

`as const` 桥接了运行时值和精确类型。

它让你只需定义一次数据（一个数组/对象），然后从中派生出精确的字面量类型 — 这对于 action 类型、路由映射、配置和任何必须保持字面量而不扩展为 `string` 的判别联合标签都是无价的。