TypeScript 联合类型和类型保护

只有联合类型存在的情况下，才需要类型保护。类型保护又叫类型守护。

联合类型

interface Waiter {
  anjiao: boolean;
  say: () => {};
}

interface Teacher {
  anjiao: boolean;
  skill: () => {};
}

// animal 既可以是 Waiter 又可以是 Teacher，这就是联合类型
function judgeWho(animal: Waiter | Teacher) {}

四种类型保护

// 断言 as 语法
function judgeWho(animal: Waiter | Teacher) {
  if (animal.anjiao) {
    (animal as Teacher).skill();
  }else{
    (animal as Waiter).say();
  }
}

// in 语法
function judgeWhoTwo(animal: Waiter | Teacher) {
  if ("skill" in animal) {
    animal.skill();
  } else {
    animal.say();
  }
}

// typeof 语法
function add(first: string | number, second: string | number) {
  if (typeof first === "string" || typeof second === "string") {
    return `${first}${second}`;
  }
  return first + second;
}

// instanceof 语法，只能用在类上
class NumberObj {
  count: number;
}
function addObj(first: object | NumberObj, second: object | NumberObj) {
  if (first instanceof NumberObj && second instanceof NumberObj) {
    return first.count + second.count;
  }
  return 0;
}

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参考：
技术胖——TypeScript从入门到精通（18. 联合类型和类型保护）