TypeScript Snippets

// Custom type guard
function isString(value: unknown): value is string {
  return typeof value === 'string';
}

// Discriminated union guard
type Result = { ok: true; data: string } | { ok: false; error: Error };

function handleResult(result: Result) {
  if (result.ok) {
    console.log(result.data);  // TypeScript knows it's string
  } else {
    console.error(result.error); // TypeScript knows it's Error
  }
}

async function fetchJSON<T>(
  url: string,
  options?: RequestInit
): Promise<T> {
  const res = await fetch(url, {
    headers: { 'Content-Type': 'application/json' },
    ...options,
  });

  if (!res.ok) {
    throw new Error(`HTTP ${res.status}: ${res.statusText}`);
  }

  return res.json() as Promise<T>;
}

// Usage — fully typed!
interface User { id: number; name: string; }
const user = await fetchJSON<User>('/api/users/1');

// Make all properties optional
type Draft = Partial<Article>;

// Make all properties required
type Complete = Required<Draft>;

// Pick specific properties
type Preview = Pick<Article, 'title' | 'image'>;

// Omit specific properties
type Safe = Omit<User, 'password' | 'token'>;

// Record type
type StatusMap = Record<string, boolean>;

// Custom: Make specific keys optional
type PartialBy<T, K extends keyof T> =
  Omit<T, K> & Partial<Pick<T, K>>;

// Custom: DeepPartial
type DeepPartial<T> = {
  [P in keyof T]?: T[P] extends object
    ? DeepPartial<T[P]>
    : T[P];
};

// Brand type helper
type Brand<T, B> = T & { __brand: B };

type UserId = Brand<string, 'UserId'>;
type PostId = Brand<string, 'PostId'>;

// Creating branded values
const userId = 'user_123' as UserId;
const postId = 'post_456' as PostId;

// Type error! Can't mix them up
function getUser(id: UserId) { /* ... */ }
getUser(postId); // TS Error!
getUser(userId); // OK