TypeScript学习笔记(1:类型断言&类型保护)

interface Bird {
  fly: boolean
  sing: () => {}
}
interface Dog {
  fly: boolean
  bark: () => {}
}

// 类型断言的方式
const tarinAnimal = (animal: Bird | Dog) => {
  if (animal.fly) {
    ;(animal as Bird).sing()
  } else {
    ;(animal as Dog).bark()
  }
}
// 用in语法来做类型保护
const trainAnimalSecond = (animal: Bird | Dog) => {
  if ('sing' in animal) {
    animal.sing()
  } else {
    animal.bark()
  }
}

// typeof做类型保护
const add = (first: string | number, second: string | number) => {
  if (typeof first === 'string' || typeof second === 'string') {
    return `${first}${second}`
  } else {
    return first + second
  }
}
// instanceof语法做类型保护
class NumberObj {
  count: number
}
const addAgain = (first: object | NumberObj, second: object | NumberObj) => {
  if (first instanceof NumberObj && second instanceof NumberObj) {
    return first.count + second.count
  }
  return 0
}