类型守护

用 类型断言 来做类型保护

function animal(animalName: Bird | Dog) {
  if (animalName.fly) {
      // 类型断言
      (animalName as Bird).sing()
  } else {
      (animalName as Dog).bark()
  }
}

用 in 来做类型保护

function animalSecond(animalNameSecond: Bird | Dog) {
  if ('sing' in animalNameSecond) {
      animalNameSecond.sing()
  } else {
      animalNameSecond.bark()
  }
}

用 typeof 来做类型保护

function add(n1: string | number, n2: string | number) {
  if (typeof n1 === 'string' || typeof n2 === 'string') {
      return `${n1}${n2}`
  }
  return n1 + n2
}

使用 interface 来做类型保护

  class NumberObj {
      public count: number

      constructor(count: number) {
          this.count = count
      }
  }

  function addSecond(first: object | NumberObj, second: object | NumberObj) {
      if (first instanceof NumberObj && second instanceof NumberObj) {
          return first.count + first.count
      }
      return 0;
  }