TypeScript之字面量类型的使用详解

type A = 1;
const x: A = 1;       // ✅ 合法
const y: A = 2;       // ❌ 报错：不能赋值 number 给类型 1

type ButtonSize = 'small' | 'medium' | 'large';

function createButton(size: ButtonSize) { ... }

createButton('medium'); // ✅
createButton('huge');   // ❌ 类型错误

let a: 'yes' = 'yes';       // ✅
let b: string = 'yes';      // ✅
a = b;                      // ❌ string 可能不等于 'yes'
b = a;                      // ✅ 'yes' 一定是 string

const a = 42;       // 推断为 42（字面量类型）
let b = 42;         // 推断为 number（更宽泛）

const s = "hello";  // 推断为 "hello"
let t = "hello";    // 推断为 string

const arr = [1, 2, 3] as const;
// 推断为 readonly [1, 2, 3] （字面量元组）

const obj = {
  type: 'success',
  code: 200,
} as const;
// 推断为 { readonly type: 'success'; readonly code: 200 }

let status: 'loading' | 'done' = 'loading';

type Action = 'increment' | 'decrement';

function reducer(action: Action) {
  if (action === 'increment') {
    // ✅ Narrowing to 'increment'
  }
}

type Shape =
  | { kind: 'circle'; radius: number }
  | { kind: 'square'; side: number };

function getArea(shape: Shape) {
  if (shape.kind === 'circle') {
    return Math.PI * shape.radius ** 2;
  }
}

const options = ['up', 'down'] as const;
type Direction = typeof options[number]; // 'up' | 'down'