javascript-typescript

JavaScript & TypeScript

Overview

Modern JavaScript (ES6+) and TypeScript patterns for building robust applications.

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TypeScript Fundamentals

Type Definitions

// Basic types
type UserId = string;
type Timestamp = number;

// Object types
interface User {
  id: UserId;
  email: string;
  name: string;
  createdAt: Timestamp;
  metadata?: Record<string, unknown>;
}

// Union types
type Status = 'pending' | 'active' | 'inactive';

// Discriminated unions
type Result<T, E = Error> =
  | { success: true; data: T }
  | { success: false; error: E };

// Generic types
interface Repository<T extends { id: string }> {
  find(id: string): Promise<T | null>;
  findAll(filter?: Partial<T>): Promise<T[]>;
  create(data: Omit<T, 'id'>): Promise<T>;
  update(id: string, data: Partial<T>): Promise<T>;
  delete(id: string): Promise<void>;
}

// Utility types
type CreateUserInput = Omit<User, 'id' | 'createdAt'>;
type UpdateUserInput = Partial<Pick<User, 'name' | 'metadata'>>;
type UserKeys = keyof User;

// Conditional types
type Nullable<T> = T | null;
type NonNullableFields<T> = {
  [K in keyof T]-?: NonNullable<T[K]>;
};

// Template literal types
type EventName = `on${Capitalize<string>}`;
type HttpMethod = 'GET' | 'POST' | 'PUT' | 'DELETE';
type ApiRoute = `/${string}`;

Type Guards

// Type predicates
function isUser(obj: unknown): obj is User {
  return (
    typeof obj === 'object' &&
    obj !== null &&
    'id' in obj &&
    'email' in obj &&
    typeof (obj as User).id === 'string'
  );
}

// Discriminated union guards
interface Dog {
  kind: 'dog';
  bark(): void;
}

interface Cat {
  kind: 'cat';
  meow(): void;
}

type Animal = Dog | Cat;

function handleAnimal(animal: Animal) {
  switch (animal.kind) {
    case 'dog':
      animal.bark(); // TypeScript knows this is Dog
      break;
    case 'cat':
      animal.meow(); // TypeScript knows this is Cat
      break;
  }
}

// Assertion functions
function assertIsString(value: unknown): asserts value is string {
  if (typeof value !== 'string') {
    throw new Error('Value must be a string');
  }
}

// Exhaustive checks
function assertNever(x: never): never {
  throw new Error(`Unexpected value: ${x}`);
}