Module System and Declaration Files

// math.ts - named exports
export const PI = 3.14159;
export const E = 2.71828;

export function add(a: number, b: number): number {
    return a + b;
}

export function multiply(a: number, b: number): number {
    return a * b;
}

export interface MathResult {
    value: number;
    operation: string;
}

// main.ts - named imports
import { PI, E, add, multiply } from './math';

console.log(PI, E);
const sum = add(5, 3);
const product = multiply(4, 2);

// Import with alias
import { add as sum, multiply as times } from './math';

// Import specific items
import { PI } from './math';

// Import type only
import type { MathResult } from './math';

// user.ts - default export
export default class User {
    constructor(public name: string) {}
    
    greet() {
        return `Hello, ${this.name}`;
    }
}

// Alternative: export after declaration
class User {
    constructor(public name: string) {}
}
export default User;

// app.ts - import default
import User from './user';
const user = new User('Alice');

// mixed.ts - both default and named exports
export default class Database {
    connect() { /* ... */ }
}

export const DB_VERSION = '1.0';
export function createConnection() { /* ... */ }

// Import both
import Database, { DB_VERSION, createConnection } from './mixed';

// Can rename default import
import DB from './mixed';

// models/user.ts
export interface User {
    id: string;
    name: string;
}

// models/product.ts
export interface Product {
    id: string;
    price: number;
}

// models/index.ts - barrel export
export { User } from './user';
export { Product } from './product';
export * from './order'; // Re-export all

// Or rename during re-export
export { User as UserModel } from './user';

// app.ts - import from barrel
import { User, Product } from './models';

// Re-export with modifications
export { User } from './user';
export type { Product } from './product'; // Type-only re-export
export { Order as PurchaseOrder } from './order';

// utils.ts
export const version = '1.0';
export function format(s: string) { return s.toUpperCase(); }
export function parse(s: string) { return s.toLowerCase(); }

// Import entire module as namespace
import * as Utils from './utils';

console.log(Utils.version);
const formatted = Utils.format('hello');

// Dynamic import (returns Promise)
async function loadModule() {
    const utils = await import('./utils');
    console.log(utils.version);
    utils.format('test');
}

// Conditional loading
if (condition) {
    import('./feature').then(module => {
        module.initialize();
    });
}

// Dynamic import with type
type UtilsModule = typeof import('./utils');
const module: UtilsModule = await import('./utils');

// commonjs.js - CommonJS module
// Pattern 1: exports object
exports.name = 'MyModule';
exports.version = '1.0';
exports.greet = function(name) {
    return `Hello, ${name}`;
};

// Pattern 2: module.exports replacement
module.exports = {
    name: 'MyModule',
    version: '1.0',
    greet(name) {
        return `Hello, ${name}`;
    }
};

// Pattern 3: Single function export
module.exports = function greet(name) {
    return `Hello, ${name}`;
};

// Pattern 4: Class export
class Database {
    connect() { /* ... */ }
}
module.exports = Database;

// Pattern 5: Mixed (avoid - confusing)
exports.helper = () => {};
module.exports.main = () => {}; // Same as exports.main

// Without esModuleInterop (old style)
import * as express from 'express';
const app = express(); // Works

// With esModuleInterop: true (recommended)
import express from 'express';
const app = express(); // Works like CommonJS

// require() in TypeScript (not recommended)
const fs = require('fs'); // Type: any
const fs: typeof import('fs') = require('fs'); // With types

// Import assignment (legacy)
import fs = require('fs');
fs.readFileSync('/path');

// Type-only import for CommonJS types
import type { Express } from 'express';
const app: Express = createExpressApp();

// source.ts - TypeScript source
export const value = 42;
export default class MyClass {}

// Compiled to CommonJS (target: "commonjs")
"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.value = void 0;
exports.value = 42;

class MyClass {
}
exports.default = MyClass;

// __esModule flag helps with interop
// TypeScript/Babel use it to detect ES modules

// Import in Node.js
const mod = require('./source');
console.log(mod.value); // 42
console.log(mod.default); // MyClass (with esModuleInterop)

// With esModuleInterop, can use:
const MyClass = require('./source').default;

// For: import { x } from './module';
// TypeScript searches in order:

// 1. Relative import './module'
./module.ts
./module.tsx
./module.d.ts
./module/package.json (check "types" field)
./module/index.ts
./module/index.tsx
./module/index.d.ts

// 2. Non-relative import 'lodash'
// From /root/src/app.ts:

/root/src/node_modules/lodash.ts
/root/src/node_modules/lodash.tsx
/root/src/node_modules/lodash.d.ts
/root/src/node_modules/lodash/package.json ("types" field)
/root/src/node_modules/lodash/index.ts
/root/src/node_modules/lodash/index.d.ts

/root/node_modules/lodash/...
/node_modules/lodash/...

// 3. Check @types
/root/src/node_modules/@types/lodash.d.ts
/root/src/node_modules/@types/lodash/index.d.ts
/root/node_modules/@types/lodash/...
/node_modules/@types/lodash/...

// package.json configuration
{
    "type": "module", // Use ESM
    "exports": {
        ".": {
            "types": "./dist/index.d.ts",
            "import": "./dist/index.js",
            "require": "./dist/index.cjs"
        },
        "./utils": {
            "types": "./dist/utils.d.ts",
            "import": "./dist/utils.js"
        }
    }
}

// With Node16/NodeNext, must use extensions
// ✗ Error - missing extension
import { x } from './module';

// ✓ Correct - with extension
import { x } from './module.js'; // .js even for .ts files!

// For directories, must use index
import { y } from './utils/index.js';

// Package imports
import pkg from 'package'; // Uses package.json "exports"
import sub from 'package/submodule'; // Must be in exports map

// tsconfig.json
{
    "compilerOptions": {
        "baseUrl": "./src",
        "paths": {
            "@/*": ["./*"],
            "@components/*": ["components/*"],
            "@utils/*": ["utils/*"],
            "@models": ["models/index"]
        }
    }
}

// Usage with path mapping
import { Button } from '@components/Button';
import { User } from '@models';
import { format } from '@utils/string';
import config from '@/config';

// Without mapping (relative paths)
import { Button } from '../../components/Button';
import { User } from '../../models/index';
import { format } from '../utils/string';
import config from './config';

// Note: Bundlers need to be configured to understand paths
// Webpack: use tsconfig-paths-webpack-plugin
// Vite: use resolve.alias

// types.d.ts - declaration file

// Declare global variable
declare const VERSION: string;
declare let currentUser: User | null;

// Declare global function
declare function $(selector: string): Element;
declare function fetch(url: string): Promise<Response>;

// Declare class
declare class EventEmitter {
    on(event: string, handler: Function): void;
    emit(event: string, ...args: any[]): void;
}

// Declare interface (no declare needed)
interface User {
    id: string;
    name: string;
}

// Declare type alias
type ID = string | number;

// Declare enum
declare enum Color {
    Red,
    Green,
    Blue
}

// Usage in .ts files (no import needed for global declarations)
console.log(VERSION);
const el = $('div');
const user: User = { id: '1', name: 'Alice' };

// lodash.d.ts - types for JavaScript library

declare module 'lodash' {
    // Named exports
    export function debounce<T extends Function>(
        func: T,
        wait: number
    ): T;
    
    export function chunk<T>(
        array: T[],
        size: number
    ): T[][];
    
    export function map<T, U>(
        array: T[],
        iteratee: (value: T) => U
    ): U[];
    
    // Default export
    const _: {
        debounce: typeof debounce;
        chunk: typeof chunk;
        map: typeof map;
    };
    
    export default _;
}

// jquery.d.ts - global + module
declare module 'jquery' {
    export = $; // Export assignment for CommonJS
}

declare const $: {
    (selector: string): JQuery;
    ajax(settings: AjaxSettings): void;
};

interface JQuery {
    addClass(className: string): JQuery;
    removeClass(className: string): JQuery;
    html(content: string): JQuery;
}

interface AjaxSettings {
    url: string;
    method?: string;
    data?: any;
}

// tsconfig.json
{
    "compilerOptions": {
        "declaration": true,          // Generate .d.ts files
        "declarationMap": true,       // Generate .d.ts.map for navigation
        "emitDeclarationOnly": false, // Emit both .js and .d.ts
        "declarationDir": "./types"   // Output directory for .d.ts
    }
}

// source.ts
export interface User {
    id: string;
    name: string;
}

export function createUser(name: string): User {
    return { id: Math.random().toString(), name };
}

export default class UserManager {
    private users: User[] = [];
    
    add(user: User): void {
        this.users.push(user);
    }
}

// Generated: source.d.ts
export interface User {
    id: string;
    name: string;
}
export declare function createUser(name: string): User;
export default class UserManager {
    private users;
    add(user: User): void;
}

// Install type definitions
// npm install --save-dev @types/node
// npm install --save-dev @types/express
// npm install --save-dev @types/react

// @types/node provides Node.js types
import * as fs from 'fs';
import * as path from 'path';

fs.readFileSync('./file.txt'); // Types available

// @types/express provides Express types
import express from 'express';
const app = express();

// Some packages include their own types
// package.json has "types" field
import lodash from 'lodash-es'; // Has built-in types

// Check if types exist: https://www.npmjs.com/~types
// Or search: npm search @types/package-name

// Override @types with local declarations
// Create: node_modules/@types/custom/index.d.ts
// Or configure typeRoots in tsconfig.json
{
    "compilerOptions": {
        "typeRoots": ["./types", "./node_modules/@types"]
    }
}

// types.d.ts - ambient declarations

// Declare module with no types available
declare module 'legacy-lib' {
    export function doSomething(value: any): any;
    export const VERSION: string;
}

// Now can import
import { doSomething, VERSION } from 'legacy-lib';

// Minimal typing for quick fixes
declare module 'no-types-available' {
    const content: any;
    export default content;
}

import lib from 'no-types-available';

// Specific module path pattern
declare module '@company/*/config' {
    interface Config {
        apiKey: string;
        endpoint: string;
    }
    const config: Config;
    export default config;
}

import config from '@company/users/config';
import config2 from '@company/products/config';

// global.d.ts - type asset imports

// CSS Modules
declare module '*.module.css' {
    const classes: { [key: string]: string };
    export default classes;
}

declare module '*.module.scss' {
    const classes: { [key: string]: string };
    export default classes;
}

// Regular CSS
declare module '*.css' {
    const content: string;
    export default content;
}

// Images
declare module '*.png' {
    const value: string;
    export default value;
}

declare module '*.jpg' {
    const value: string;
    export default value;
}

declare module '*.svg' {
    import { FC, SVGProps } from 'react';
    const content: FC<SVGProps<SVGSVGElement>>;
    export default content;
}

// JSON (if not using resolveJsonModule)
declare module '*.json' {
    const value: any;
    export default value;
}

// Usage in components
import styles from './Component.module.css';
import logo from './logo.png';
import icon from './icon.svg';
import data from './data.json';

const className = styles.container; // Type: string
const imageSrc = logo; // Type: string

// Extend existing module with new exports
declare module 'express' {
    // Add custom properties to Request
    interface Request {
        user?: {
            id: string;
            name: string;
        };
        startTime?: number;
    }
    
    // Add custom response methods
    interface Response {
        sendSuccess(data: any): void;
        sendError(message: string): void;
    }
}

// Now can use augmented types
import { Request, Response } from 'express';

app.use((req: Request, res: Response, next) => {
    req.startTime = Date.now(); // ✓ Type-safe
    if (req.user) {
        console.log(req.user.name); // ✓ Type-safe
    }
    res.sendSuccess({ ok: true }); // ✓ Type-safe
});

// Augment module from within a module
// my-plugin.ts
import 'express';

declare module 'express' {
    interface Request {
        customField: string;
    }
}

// Available after import
import './my-plugin';
app.get('/', (req, res) => {
    console.log(req.customField); // ✓ Available
});

// extensions.d.ts - module file with global augmentation

// Must have at least one import/export to be a module
export {};

// Augment global scope
declare global {
    // Add to Window interface
    interface Window {
        myApp: {
            version: string;
            config: AppConfig;
        };
        gtag: (command: string, ...args: any[]) => void;
    }
    
    // Add global variable
    var APP_ENV: 'development' | 'production';
    
    // Add to Array prototype
    interface Array<T> {
        firstOrNull(): T | null;
        lastOrNull(): T | null;
    }
    
    // Add to String prototype
    interface String {
        toKebabCase(): string;
        toCamelCase(): string;
    }
}

// Usage anywhere in project
window.myApp.version; // ✓ Type-safe
console.log(APP_ENV); // ✓ Type-safe

const arr = [1, 2, 3];
const first = arr.firstOrNull(); // ✓ Type-safe

const str = 'hello-world';
const kebab = str.toKebabCase(); // ✓ Type-safe

// types.d.ts
interface User {
    id: string;
    name: string;
}

// main.ts - reference other file
/// <reference path="./types.d.ts" />

const user: User = { id: '1', name: 'Alice' };

// Better modern approach: use import
import type { User } from './types';

// Multiple references
/// <reference path="./globals.d.ts" />
/// <reference path="./utils.d.ts" />
/// <reference path="./models.d.ts" />

// Note: Triple-slash directives must be at top of file
// Before any code or imports

// Include types from @types package
/// <reference types="node" />

// Now have Node.js types without explicit import
const fs: typeof import('fs') = require('fs');
console.log(__dirname); // Global Node.js variable

// Include jQuery types
/// <reference types="jquery" />

$('div').addClass('active'); // jQuery available globally

// In declaration files (.d.ts)
/// <reference types="react" />

declare module 'my-react-library' {
    import { ComponentType } from 'react';
    export const MyComponent: ComponentType<{}>;
}

// Modern approach in tsconfig.json
{
    "compilerOptions": {
        "types": ["node", "jest", "jquery"]
    }
}

// Reference specific library versions
/// <reference lib="es2015" />
/// <reference lib="dom" />

// Enables specific APIs
const map = new Map(); // ES2015
const el = document.querySelector('div'); // DOM

// Reference newer libs
/// <reference lib="es2020.promise" />
/// <reference lib="es2021.string" />

// Promise.allSettled (ES2020)
const results = await Promise.allSettled([
    Promise.resolve(1),
    Promise.reject('error')
]);

// String.replaceAll (ES2021)
const replaced = 'hello'.replaceAll('l', 'L');

// Available lib values:
// es5, es2015, es2016, es2017, es2018, es2019, es2020, es2021
// dom, dom.iterable, webworker, scripthost
// es2015.core, es2015.collection, es2015.promise, etc.

// Modern approach in tsconfig.json
{
    "compilerOptions": {
        "lib": ["es2020", "dom", "dom.iterable"]
    }
}

// custom-lib.d.ts - custom library definitions
/// <reference no-default-lib="true" />

// Provide minimal environment (rare use case)
interface Array<T> {
    length: number;
    push(...items: T[]): number;
}

interface String {
    length: number;
    substring(start: number, end?: number): string;
}

// Combined directives
/// <reference lib="es2020" />
/// <reference types="node" />
/// <reference path="./custom.d.ts" />

// Order matters: must be at very top
/// <reference path="./types.d.ts" />
/// <reference types="node" />
import { readFile } from 'fs'; // After directives

// Directive ignored if not at top
import something from './module';
/// <reference types="node" /> // ✗ Ignored - comes after import