charlypoly

interface Person {
  name: string
  address?: {
    city: string
    zipcode: string
  }
}

const person: Person = { name: "John", address: undefined }

charlypoly

interface Person {
  name: string
  address?: {
    city: string
    zipcode: string
  }
}

const p: Person = { name: "John", address: undefined }

charlypoly

interface Person {
  name: string
  address?: {
    city: string
    zipcode: string
  }
}

const p: Person = { name: "John" }

charlypoly

let myPrices: number[] = [1, 2.4, 2.0]

const defaultPrice = myPrices[40]
// `defaultPrice` is of type `number | undefined`

charlypoly

let myPrices: number[] = [1, 2.4, 2.0]

const defaultPrice = myPrices[40]
// `defaultPrice` is of type `number`

charlypoly

function isPremiumUser(user: User | PremiumUser): user is PremiumUser {
  return user.plan === "premium"
}

let user: PremiumUser | User | undefined

// the line below should raise a TypeScript error,
//  but it doesn't!
if (isPremiumUser(user)) {
  // ...

charlypoly

interface Success {
  type: `${string}Success`;
  body: string;
}

interface Error {
  type: `${string}Error`;
  message: string;
}

charlypoly

type UserArg = PremiumUser | USer

function processUSer({ plan, premiumOptions }: UserArg) {
  if (plan === "premium") {
    premiumOptions
    // `premiumOptions` is of type `PremiumOptions`
  } else {
    premiumOptions
    // `premiumOptions` is of type `undefined`
  }

charlypoly

type UUID = string;

interface BasicUser {
   id: UUID;
   first_name: string;
   last_name: string;
}

interface User extends BasicUser {
   plan: 'free'

charlypoly

import { isString } from "lodash"

let name: string | undefined = "John"
// `name` is of type `string | undefined`

let price: number | undefined = 1
// `price` is of type `number | undefined`

if (isString(name)) {
  name