Typescript cheatsheet

ts.md

Core types

// enum
enum Role {
  ADMIN,
  READ_ONLY,
  AUTHOR
}

// object
type Person = {
  readonly id: string | number; // readonly, cannot be re-assigned
  firstName: string; // string
  lastName: string; // string
  fullName: (firstName: string, lastName: string ) => string; // function with two params string and returns string
  age: number; // number
  month: number | string; // union
  hobbies: string[]; // array
  childBornAt: (number | string)[]; // array of string or number
  keyValuePair: [string, number]; // turple: we are sure the first element is a string and the second number
  role: Role;
  callback: (num: number) => void; // function that doesnt return anything
}

Interfaces

For objects this is preferable

interface Indexed {
  readonly id: string | number;
}

interface Named {
  firstName: string; // string
  middleName?: string; // optional
  lastName: string; // string
}

interface Greetable extends Named, Indexed {
  greet: (name: string) => string
}

// use it like so
const Manel: Greetable = {
  id: 123,
  firstName: 'Manel',
  lastName: 'Paiva',
  greet: (name) => `Hello ${name}`
}

Can also be used for function types

interface AddFn {
  (a: number, b: number): number
}
const add : AddFn = (a, b) => a + b

Intersection types

type Admin = {
  name: string
  priviliges: string[]
}

type Employee = {
  name: string
  startDate: Date
}

// in objects combines them
type ElevatedUser = Employee & Admin

const admin: ElevatedUser = {
  name: 'Joao',
  priviliges: ['delete users'],
  startDate: new Date()
}

type Combinable = string | number
type Numeric = number | boolean

// in union types intersects whats in common
type Universal = Combinable & Numeric // number

Type guards

function add(a: Combinable, b: Combinable) {
  // check if its a string before adding
  if (typeof a === 'string' || typeof b === 'string') {
    return a.toString() + b.toString()
  }
  return a + b
}

Objects

type UnknownEmployee = Employee | ElevatedUser

function printEmployeeInformation(emp: UnknownEmployee) {
  console.log('Name: ', emp.name)
  // check if the UnknownEmployee has the priviliges key
  if ('priviliges' in emp) {
    console.log('Privileges: ', emp.priviliges)
  }
}

Class instances

class Car {
  drive() {
    console.log('Driving...')
  }
}

class Truck {
  drive() {
    console.log('Driving...')
  }
  loadCargo(ammount: number): void {
    console.log('Loading: ', ammount)
  }
}

type Vehicle = Car | Truck
const mercedez = new Car()
const man = new Truck()

function useVehicle(vehicle: Vehicle) {
  vehicle.drive()
  // check if the vehicle is an instance of truck
  if (vehicle instanceof Truck) vehicle.loadCargo(2000)
}