Object Types

• Ex - Array<T>, Map<K, V>, Set<T>, and Promise<T>

// way 1 - {} function foo(x: {a: number; b: number}): number { // code } // way 2 - interface interface XInterface { a: number; b: number; } function foo(x: XInterface) { // code }

// way 3 - Type alias type xType = { a: number; b: number; }; function foo(x: xType) { // code }

undefined

// avoid undefined for optional property
// default value ->  y = 0
interface fooInterface {
  x: number;
  y?: number;
}

function foo({x, y = 0}: fooInterface) {
  return x + y;
}

Readonly

interface xInterface {
  readonly x: number;
}

function foo(obj: fooInterface) {
  obj.x = 34; // error
}

Extending

interface A { x: number; } interface B { y: number; } interface Combo extends A, B {}

interface A { x: number; } interface B { y: number; } // & = intersection // almost same as extends // difference is how error is handled type Combo = A & B;

Generic

// interface interface xyInterface<T> { x: T; y: T; } // usage let num: xyInterface<number> = { x: 3, y: 6, };

// type alias type xyType<T> = { x: T; y: T; }; // we can use union type xyOrNull<T> = xyType<T> | null; // usage let num: xyOrNull<number> = { x: 3, y: 6, }; let num1: xyOrNull<null> = { x: null, y: null, }; let num2: xyOrNull<null> = null;