TS samples

generics.ts

function fail(arr: never): never {
  return new Error;
}

function pluck<T, K extends keyof T>(obj: T, propertyNames: K[]): T[K][] {
  return propertyNames.map(key => obj[key])
}

interface Author {
  name: string;
}

interface Course = {
  id: number;
  title: string;
  author: number;
}

const course: Course = {
  id: 123,
  title: 'Advanced Typescript',
  author: {
    name: 'Marek Dano'
  },
}

pluck(course, ['name]) // error
pluck(course, ['title]) // OK
                                           
type Freeze<T> = {
  readonly [P in keyof T]: T[P];
}

const freezeObj = Freeze<Course> = course;
// OR use built-in function Readonly
const freezeObj = Readonly<Course> = course;
freezeObj.title = ''; // error - can't change property

interface Mentor {
  readonly name: string;
  readonly course: Course;
}

type Writable<T extends {[key: string]: any}, K extends string> = {
  [P in K]: T[P]
}

const writableMentor: Writable<Mentor, keyof Mentor> = {name: 'davy', course};
writableMentor.course = course; // works OK

// OR use
type Writable<T> = {
  -readonly [P in keyof T]: T[P];
}

const writableMentor: Writable<Mentor> = {name: 'davy', course};
writableMentor.course = course; // works OK
      
// Overloading functions
export function findCourse(name: string): Course[];
export function findCourse(id: number): Course[];
// above this can be replaced by
export function findCourse <T>(args: T): Course[];
export function findCourse (author: Author): Course[];

export function find<T, U>(args: T): U[] // example of using generics

export function findCourse(searchTerm: number | string | Author): Course[] {
  if (typeof searchTerm === 'string') {
    return courses.filter(course => course.title === searchTerm);
  }  
  if (typeof searchTerm === 'number') {
    return courses.filter(course => course.id === searchTerm);
  }
  
  return courses.filter(course => course.author.name.includes(searchTerm)); 
}

findCourse<number>(123);
findCourse<string>('Advanced Typescript');

// conditional type below
type NoNullable<T> = T extends null | undefined ? never : T;
function transform<T extends string>(text: T): T extends string ? string : null;

function transform(text: string | null): string | null {
  return text && text.replace(/f/g, 'p');
}

interface Cat {}
interface Lion {}
interface Sharp {
  swim: boolean;
}

type Animal = Cat | Lion | Shark;
type ExtractFish<A> = A extends { swim: boolean } ? A : never;
type Fish = ExtractFish<Animal>;