Created : August 12, 2020

Learned from TypeScript official tutorial.

  • TypeScript adds its own layer on top of JavaScript. This layer is the TypeScript type system.

Types by Inference

  • TypeScript builds a type-system which accepts JavaScript code but has types.
  • This offers a type-system without needing to add extra characters to make types explicit in the code.
  • Just by let helloWorld = 'Hello World';, TypeScript knows that helloWorld is a string.

Defining Types

  • Two syntaxes for building types: interface and type

    • Should prefer interface, and use type when you need specific features.
  • interface declaration : An explicit way to describe this object’s shape.
interface User {
  name: string;
  id: number;

const user: User = {
  name: "Hayes",     // cannot use other name for property like 'username'
  id: 0,
  • interface declaration can also be used with classes.
interface User {
  name: string;
  id: number;

class UserAccount {
  name: string;
  id: number;

  constructor(name: string, id: number) {
    this.name = name;
    this.id = id;

const user: User = new UserAccount("Murphy", 1);
  • TypeScript has more primitive types than JavaScript.

    • any : allow anything
    • unknown : ensure someone using this type declares what the type is
    • never : not possible that this type could happen
    • void : a function which returns undefined or has no return value

Composing Types


Declare that a type could be one of many types.

// Describe a boolean type as being either true or false.
type MyBool = true | false;

// Describe a set of strings or numbers literal.
type WindowStates = "open" | "closed" | "minimized";
type LockStates = "locked" | "unlocked";
type OddNumbersUnderTen = 1 | 3 | 5 | 7 | 9;

// Function accepts an array or a string.
function getLength(obj: string | string[]) {
  return obj.length;

// Use 'typeof' to differentiate between a string and an array.
function wrapInArray(obj: string | string[]) {
  if (typeof obj === "string") {
    return [obj];
  } else {
    return obj;


e.g. for array,

  • An array without generics could contain anything.
  • An array with generics can describe what values are inside in the array.
type StringArray = Array<string>;     // only strings are allowed in the array
type NumberArray = Array<number>;
type ObjectWithNameArray = Array<{ name: string }>;

// Declare your own types which use generics.
interface Backpack<Type> {
  add: (obj: Type) => void;
  get: () => Type;

declare const backpack: Backpack<string>;

// 'object' is a string
const object = backpack.get();

backpack.add(23);       // Cannot pass a number

Structural Type System

  • duck typing or structural typing : Type checking focuses on the shape which values have.
  • If two objects have the same shape, they are considered the same.
interface Point {
  x: number;
  y: number;

function printPoint(p: Point) {
  console.log(`${p.x}, ${p.y}`);

const point = { x: 12, y: 26 };
printPoint(point);             // "12, 26"
  • The point variable is never declared to be a Point type, but TypeScript compares the shape of point to the shape of Point in the type-check. Because they both have the same shape, then it passes.