TypeScript as const 怎么让对象和数组变为字面量类型？

Question

仓乃文 · Accepted Answer

普通对象和数组的类型会被拓宽（"hello" 变成 string，1 变成 number），无法用于精确的联合类型约束。as const 断言将值收窄为最精确的字面量类型，所有属性变为 readonly，适合定义常量配置和精确类型。

数组变只读元组：
```typescript
const roles = ["admin", "editor", "viewer"] as const;
// 类型：readonly ["admin", "editor", "viewer"]
// 而非 string[]
type Role = typeof roles[number];  // "admin" | "editor" | "viewer"
```

对象变只读字面量：
```typescript
const ROUTES = {
  home: "/",
  login: "/login",
  dashboard: "/dashboard"
} as const;
// 类型：{ readonly home: "/"; readonly login: "/login"; readonly dashboard: "/dashboard" }
type Route = typeof ROUTES[keyof typeof ROUTES];  // "/" | "/login" | "/dashboard"
```

配合 switch 实现穷尽检查：
```typescript
type Status = "loading" | "success" | "error";
function handle(status: Status) {
  switch (status) {
    case "loading": return "...";
    case "success": return "OK";
    case "error": return "ERR";
  }
}
```

补充说明：
- as const 等价于给所有属性加 readonly + 字面量类型
- 不能对变量声明用 as const（let/const 声明本身已有推导）
- as const 可以用在表达式上：[1, 2, 3] as const
- TS 5.0+ 支持 as const 参数推断（无需手动断言）
- 适合定义路由表、权限枚举、状态机等常量