typescript-generics

// Source: https://www.typescriptlang.org/docs/handbook/2/generics.html
function first<T>(array: T[]): T | undefined {
  return array[0];
}

const num = first([1, 2, 3]);       // number | undefined
const str = first(['a', 'b']);      // string | undefined

// Source: https://www.typescriptlang.org/docs/handbook/2/generics.html
interface HasId {
  id: string;
}

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

// Works with any object that has 'id'
const user = findById(users, '123');
const post = findById(posts, '456');

// Source: https://www.typescriptlang.org/docs/handbook/2/generics.html
interface ListProps<T> {
  items: T[];
  renderItem: (item: T) => React.ReactNode;
  keyExtractor: (item: T) => string;
}

function List<T>({ items, renderItem, keyExtractor }: ListProps<T>) {
  return (
    <ul>
      {items.map(item => (
        <li key={keyExtractor(item)}>{renderItem(item)}</li>
      ))}
    </ul>
  );
}

// Usage with full type inference
<List
  items={users}
  renderItem={(user) => user.name}  // user is typed as User
  keyExtractor={(user) => user.id}
/>

// Source: https://www.typescriptlang.org/docs/handbook/2/conditional-types.html
type ApiResponse<T> = T extends undefined
  ? { success: true }
  : { success: true; data: T };

// Result: { success: true }
type VoidResponse = ApiResponse<undefined>;

// Result: { success: true; data: User }
type UserResponse = ApiResponse<User>;

// Source: https://www.typescriptlang.org/docs/handbook/2/generics.html
function map<T, U>(array: T[], fn: (item: T) => U): U[] {
  return array.map(fn);
}

// T = User, U = string
const names = map(users, user => user.name);

// Merge objects
function merge<T, U>(obj1: T, obj2: U): T & U {
  return { ...obj1, ...obj2 };
}

// Source: https://www.typescriptlang.org/docs/handbook/2/generics.html
interface PaginatedResult<T = unknown> {
  data: T[];
  page: number;
  total: number;
}

// Uses default
const result: PaginatedResult = { data: [], page: 1, total: 0 };

// Explicit type
const users: PaginatedResult<User> = { data: [], page: 1, total: 0 };

// Source: https://www.typescriptlang.org/docs/handbook/utility-types.html
// Blocks inference to ensure parameter matches previously inferred type
function createStreetLight<C extends string>(
  colors: C[],
  defaultColor?: NoInfer<C>
) {
  return { colors, defaultColor };
}

createStreetLight(['red', 'yellow', 'green'], 'red');    // OK
createStreetLight(['red', 'yellow', 'green'], 'blue');   // Error