type guard 型別判斷

最基本的類型可以用 typeof 做判斷

function getLength(something: string | number): number {
  if (typeof something === 'number') {
    return something.toString().length
  }
  return something.length
}

console.log(getLength('123'))
console.log(getLength(123))

custom type 可以用 in 來做判斷是否為某個 type

type Employee = {
  name: string
  department: string
}
type Leader = {
  name: string
  department: string
  authority: boolean
}
type UnknownEmployee = Employee | Leader

function work(someone: UnknownEmployee) {
  if ('authority' in someone) {
    console.log(someone.authority)
    return
  }
  console.log('not leader')
}

const employee: Employee = {
  name: 'Tom',
  department: 'eng'
}
const leader: Leader = {
  name: 'Tom',
  department: 'eng',
  authority: true
}

work(employee) // not leader
work(leader) // true

寫成 class 裡面的 method 同理，可以用 in

class SWE {
  coding() {
    console.log('coding')
  }
}
class Manager {
  meeting() {
    console.log('meeting')
  }
}
type Employee = SWE | Manager

function work(employee: Employee) {
  if ('coding' in employee) {
    employee.coding()
  }
  if ('meeting' in employee) {
    employee.meeting()
  }
}
work(new SWE())

class 可以用 instanceof做判斷注意：type 不能用 instanceof

class SWE {
  coding() {
    console.log('coding')
  }
}

class Manager {
  meeting() {
    console.log('meeting')
  }
}

type Employee = SWE | Manager

function work(employee: Employee) {
  if (employee instanceof SWE) {
    employee.coding()
  }
  if (employee instanceof Manager) {
    employee.meeting()
  }
}

work(new SWE())