Explain the module augmentation pattern in TypeScript.

Module augmentation extends existing modules by declaring additional properties or methods. Basic Module Augmentation: // Extending global interfaces declare global { interface Window { myCustomProperty: string; myCustomMethod(): void; } } // Now you can use it window.myCustomProperty = 'Hello'; window.myCustomMethod = () => console.log('Custom method'); Augmenting Third-Party Libraries: // Extending Express Request interface import { Request } from 'express'; declare module 'express-serve-static-core' { interface Request { user?: { id: string; email: string; }; sessionId?: string; } } // Now you can use req.user with type safety app.use((req, res, next) => { req.user = { id: '123', email: 'user@example.com' }; next(); }); Augmenting Node.js Global: // Adding custom properties to Node.js global declare global { namespace NodeJS { interface Global { myAppConfig: { apiUrl: string; version: string; }; } } } // Usage global.myAppConfig = { apiUrl: 'https://api.example.com', version: '1.0.0' }; Augmenting Existing Modules: // Extending Array prototype declare global { interface Array { first(): T | undefined; last(): T | undefined; } } Array.prototype.first = function(this: T[]): T | undefined { return this[0]; }; Array.prototype.last = function(this: T[]): T | undefined { return this[this.length - 1]; }; // Usage with type safety const numbers = [1, 2, 3]; const first = numbers.first(); // Type: number | undefined const last = numbers.last(); // Type: number | undefined Library-Specific Augmentation: // Extending jQuery (if using) declare module 'jquery' { interface JQuery { customPlugin(options?: any): JQuery; } } // Extending Lodash import { LoDashStatic } from 'lodash'; declare module 'lodash' { interface LoDashStatic { customUtility(input: T): T; } } Benefits: • Add type safety to existing libraries • Extend global objects safely • Maintain compatibility with third-party code • Enable custom functionality with type checking.