TypeScript Advanced

Generics with Constraints

interface HasId {
  id: string;
}

function findById<T extends HasId>(items: T[], id: string): T | undefined {
  return items.find(item => item.id === id);
}

function groupBy<T, K extends string | number>(
  items: T[],
  keyFn: (item: T) => K
): Record<K, T[]> {
  return items.reduce((acc, item) => {
    const key = keyFn(item);
    (acc[key] ??= []).push(item);
    return acc;
  }, {} as Record<K, T[]>);
}

type ApiResponse<T> = {
  data: T;
  meta: { page: number; total: number };
};

async function fetchApi<T>(url: string): Promise<ApiResponse<T>> {
  const res = await fetch(url);
  return res.json();
}

Conditional Types

type IsString<T> = T extends string ? true : false;

type Flatten<T> = T extends Array<infer U> ? U : T;

type UnwrapPromise<T> = T extends Promise<infer U> ? UnwrapPromise<U> : T;

type FunctionReturn<T> = T extends (...args: any[]) => infer R ? R : never;

type ExtractRouteParams<T extends string> =
  T extends `${string}:${infer Param}/${infer Rest}`
    ? Param | ExtractRouteParams<Rest>
    : T extends `${string}:${infer Param}`
      ? Param
      : never;

type Params = ExtractRouteParams<"/users/:userId/posts/:postId">;

Mapped Types

type Readonly<T> = { readonly [K in keyof T]: T[K] };

type Optional<T> = { [K in keyof T]?: T[K] };

type Nullable<T> = { [K in keyof T]: T[K] | null };

type PickByType<T, V> = {
  [K in keyof T as T[K] extends V ? K : never]: T[K];
};

interface User {
  id: string;
  name: string;
  age: number;
  active: boolean;
}

type StringFields = PickByType<User, string>;

type EventMap<T> = {
  [K in keyof T as `on${Capitalize<string & K>}`]: (value: T[K]) => void;
};

type UserEvents = EventMap<{ login: User; logout: string }>;

Discriminated Unions

type Result<T, E = Error> =
  | { ok: true; value: T }
  | { ok: false; error: E };