LANGUAGE » JAVASCRIPT

Typescript

Last updated:
Reference:www.typescriptlang.org

Overview ​

TypeScript offers all of JavaScript’s features, and an additional layer on top of these: TypeScript’s type system.

Run type check ​

shell
tsc --noEmit

Types ​

Primitives ​

  • Any: any
  • Number: number bigint
  • String: string
  • Boolean: boolean
  • List: Array []
  • Dict: Record<Type1, Type2>
  • Set: -
  • Symbol: symbol [1]
  • Function: Function
  • Empty: null undefined void (function which returns undefined or has no return value)
  • Unset: unknown (someone must declare its type)
  • Never: never
  • Union: |
  • Class: -
  • Final: -

Async functions don't have a primitive type. Declare one with:

ts
type AsyncFunction = () => Promise<any>;

To check the type, use typeof:

ts
typeof n === 'number'
Array.isArray(a)

Type from dict ​

Useful if you have an object and want to create a type from its keys.

ts
const point = { x: 10, y: 20 };
type CoordXY = keyof typeof point;  // CoordXP = 'x' | 'y'

Usage ​

For variables:

ts
const hello: string = 'Hello World';
const value: number = 10;
const dict1: {[key: string]: string} = {};
const dict2: Record<string, string> = {};
const ids: number[] = [0, 1];

For functions:

ts
function getAdminUser(): User {
  // Must return a value of type User
}

For classes:

ts
class UserAccount {
  id: number;
  name: string;

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

const newuser: User = new UserAccount(1, 'Kai');

Expanding arguments:

ts
function sum({ a, b }: {a: number, b: number} {
  return a + b;
}

Custom types / interfaces ​

Decide which values are accepted:

ts
type MyBool = true | false;
type WindowStates = 'open' | 'closed' | 'minimized';
type PositiveOddNumbersUnderTen = 1 | 3 | 5 | 7 | 9;

If the object is complex, an interface will be needed:

ts
interface User {
  id: number;
  name: string;
}

const user: User = {
  id: 0,
  name: 'Sora',
};

If the key can be a whole type, use:

ts
interface Parameters {
  [key: string]: any;
}

Generic types (like Array):

ts
type NumberArray = Array<number>;
type ObjectWithNameArray = Array<{ name: string }>;

Create generic types using Type:

ts
interface Backpack<Type> {
  add: (obj: Type) => void;
  get: () => Type;
}
declare const backpack: Backpack<string>;
const object = backpack.get();  // object is string
backpack.add('Notebook');       // Must pass a string as parameter

Exporting/importing types ​

ts
export type { User };
import type { User } from '../types/user';

Operators ​

Non-null assertion operator: !

ts
stringOrNull!.toLowerCase();

Typings ​

If any other file type other than .ts or .tsx is imported, typescript does not recognize it. Ex:

ts
import styles from './styles.module.css';

To solve this, create a file typings.d.ts containing:

js
declare module '*.module.css';

Tips ​

Object.keys ​

Object.keys() return value by default is any. To change it to its keys instead, use:

ts
(Object.keys(mydict) as Array<keyof typeof mydict>).filter(key => ...);

key will have the type of keyof typeof mydict.