Typescript: 他實際上沒有驗證你的型別

const justAFunction = (n: number): string => {
  return `${n}`
}

console.log(justAFunction)

"use strict";
const justAFunction = (n) => {
    return `${n}`;
};
console.log(justAFunction);

檢查型別

const justAFunction = (str: string[] | string): string => {
  return str.join(' ')
}

console.log(justAFunction(["Hello", "World"]))
console.log(justAFunction("Hello World"))

index.ts:2:14 - error TS2339: Property 'join' does not exist on type 'string | string[]'.
  Property 'join' does not exist on type 'string'.

2   return str.join(' ')
               ~~~~

Found 1 error in index.ts:2

const justAFunction = (str: string[] | string): string => {
  if (typeof str === 'string') {
    return str
  }

  return str.join(' ')
}

console.log(justAFunction(["Hello", "World"]))
console.log(justAFunction("Hello World"))

讓外部資料對照到你的型別

{
  "firstname": "John",
  "lastname": "Doe",
  "birthday": "1985-04-03"
}

interface User {
  firstname: string
  lastname: string
  birthday: string
}

const retrieveUser = async (): Promise<User> => {
  const resp = await fetch('/user/me')
  return resp.json()
}

const validate = (obj: any): obj is User => {
  return obj !== null
    && typeof obj === 'object'
    && 'firstname' in obj
    && 'lastname' in obj
    && 'birthday' in obj
    && typeof obj.firstname === 'string'
    && typeof obj.lastname === 'string'
    && typeof obj.birthday === 'string'
}

const user = await retrieveUser()

if (!validate(user)) {
  throw Error("User data is invalid")
}

進入 Zod

import { z } from 'zod'

const User = z.object({
    firstname: z.string(),
    lastname: z.string(),
    birthday: z.string()
  })

const UserType = z.infer<User>

const userResp = await retrieveUser()
const user = User.parse(userResp)

結論