TypeScript

JavaScript libraries

TypeScript is just like ES2015 with type-checking. All ES2015 (classes, etc) should work.

Basic types

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any
void

boolean
number
string

null
undefined

string[]          /* or Array<string> */
[string, number]  /* tuple */

string | null | undefined   /* union */

never  /* unreachable */
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enum Color {Red, Green, Blue = 4}
let c: Color = Color.Green

Declarations

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let isDone: boolean
let isDone: boolean = false
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function add (a: number, b: number): number {
  return a + b
}

// Return type is optional
function add (a: number, b: number) { ... }

Type assertions

Variables

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let len: number = (input as string).length
let len: number = (<string> input).length  /* not allowed in JSX */

Functions

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function object(this: {a: number, b: number}, a: number, b: number) {
  this.a = a;
  this.b = b;
  return this;
}

// this is used only for type declaration
let a = object(1,2);
// a has type {a: number, b: number}

Interfaces

Inline

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function printLabel (options: { label: string }) {
  console.log(options.label)
}

// Note the semicolon
function getUser (): { name: string; age?: number } {
}

Explicit

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interface LabelOptions {
  label: string
}

function printLabel(options: LabelOptions) { ... }

Optional properties

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interface User {
  name: string,
  age?: number
}

Read only

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interface User {
  readonly name: string
}

Dynamic keys

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{
  [key: string]: Object[]
}

Type aliases

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type Name = string | string[]

Function types

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interface User { ... }

function getUser(callback: (user: User) => any) { callback({...}) }

getUser(function (user: User) { ... })

Classes

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class Point {
  x: number
  y: number
  static instances = 0
  constructor(x: number, y: number) {
    this.x = x
    this.y = y
  }
}

Inheritance

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class Point {...}

class Point3D extends Point {...}

interface Colored {...}

class Pixel extends Point implements Colored {...}

Short fields initialisation

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class Point {
  static instances = 0;
  constructor(
    public x: number,
    public y: number,
  ){}
}

Fields which do not require initialisation

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class Point {
  public someUselessValue!: number;
  ...
}

Generics

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class Greeter<T> {
  greeting: T
  constructor(message: T) {
    this.greeting = message
  }
}

let greeter = new Greeter<string>('Hello, world')

Modules

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export interface User { ... }

Type extraction

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interface Building {
  room: {
    door: string,
    walls: string[],
  };
}

type Walls = Building['room']['walls']; // string[]