TypeScript学习笔记之类型缩小

function printID(id: string|number) {
  if (typeof id === "string") {
    console.log(id.toUpperCase())
  } else {
    console.log(id)
  }
}

// 测试
printID(123)
printID("aaa")

type Direction = "left" | "right" | "top" | "bottom"
function printDirection(direction: Direction) {
  // 平等类型缩小
  switch (direction) {
    case "left":
      console.log(direction)
      break
    case "right":
      console.log(direction)
      break
    case "top":
      console.log(direction)
      break
    case "bottom":
      console.log(direction)
      break
    default:
      console.log("调用默认方法")
  }
}

// 测试
printDirection("left")
printDirection("right")
printDirection("top")
printDirection("bottom")

function printTime(time: string|Date) {
  // 判断time是否是Date的实例
  if (time instanceof Date) {
    console.log(time.toUTCString())
  } else {
    console.log(time)
  }
}

// 测试
printTime("2020-01-02")
const date = new Date()
printTime(date)

class Teacher {
  working() {
    console.log("正在工作")
  }
}

class Student {
  studying() {
    console.log("正在学习")
  }
}

function work(p: Student | Teacher) {
  // 判断是哪一个实例
  if (p instanceof Teacher) {
    p.working()
  } else {
    p.studying()
  }
}

// 测试
const stu = new Student()
const tec = new Teacher()

work(stu) // 正在学习
work(tec) // 正在工作

// () => void表示是一个函数类型
type Dog = {running: () => void}
type Fish = {swimming: () => void}

function walk(animal: Dog|Fish) {
  // 判断函数是否在animal中
  if ("swimming" in animal) {
    animal.swimming()
  } else {
    animal.running()
  }
}

// 测试
const dog: Dog = {
  running() {
    console.log("狗是跑的")
  }
}
const fish: Fish = {
  swimming() {
    console.log("鱼是游的")
  }
}

walk(dog) // 狗是跑的
walk(fish) // 鱼是游的