TypeScript中穷尽性检查的实用技巧

type NumberOrString = number | string;

type Status = 'idle' | 'loading' | 'success' | 'error';

function handleStatus(status: Status) {
  switch (status) {
    case 'idle':
      console.log('Status is idle');
      break;
    case 'loading':
      console.log('Status is loading');
      break;
    case 'success':
      console.log('Status is success');
      break;
    // 如果遗漏了 'error' 分支，TypeScript会发出警告
  }
}

type Shape = 
  | { kind: 'circle'; radius: number }
  | { kind: 'square'; sideLength: number }
  | { kind: 'triangle'; base: number; height: number };

function isCircle(shape: Shape): shape is { kind: 'circle'; radius: number } {
  return shape.kind === 'circle';
}

function isSquare(shape: Shape): shape is { kind: 'square'; sideLength: number } {
  return shape.kind === 'square';
}

function isTriangle(shape: Shape): shape is { kind: 'triangle'; base: number; height: number } {
  return shape.kind === 'triangle';
}

function calculateArea(shape: Shape): number {
  if (isCircle(shape)) {
    return Math.PI * shape.radius ** 2;
  } else if (isSquare(shape)) {
    return shape.sideLength ** 2;
  } else if (isTriangle(shape)) {
    return 0.5 * shape.base * shape.height;
  }
  // 如果所有类型守卫都返回false，TypeScript会认为这里不可达
  // 但为了满足严格性，可以添加一个默认处理或抛出错误
  const exhaustiveCheck: never = shape;
  throw new Error(`Unhandled shape kind: ${(shape as any).kind}`);
}

type Direction = 'north' | 'south' | 'east' | 'west';

function logDirection(direction: Direction) {
  switch (direction) {
    case 'north':
      console.log('Going north');
      break;
    case 'south':
      console.log('Going south');
      break;
    case 'east':
      console.log('Going east');
      break;
    case 'west':
      console.log('Going west');
      break;
    default:
      // 捕获所有可能的值，但Direction联合类型已经穷尽
      const exhaustiveCheck: never = direction;
      throw new Error(`Unknown direction: ${exhaustiveCheck}`);
  }
}

function formatInput(input: string): string;
function formatInput(input: number): string;
function formatInput(input: boolean): string;
function formatInput(input: string | number | boolean): string {
  if (typeof input === 'string') {
    return input.toUpperCase();
  } else if (typeof input === 'number') {
    return input.toString();
  } else if (typeof input === 'boolean') {
    return input.toString();
  }
  const exhaustiveCheck: never = input;
  throw new Error(`Unhandled input type: ${typeof exhaustiveCheck}`);
}