# Install the types for Node.js built-ins
npm install --save-dev @types/node

# Install types for Express
npm install express
npm install --save-dev @types/express

# Install types for lodash
npm install lodash
npm install --save-dev @types/lodash

# Install types for Jest
npm install --save-dev jest @types/jest

// tsconfig.json
{
  "compilerOptions": {
    // Only include these specific @types packages globally
    "types": ["node", "jest"],

    // Search for @types in these directories instead of node_modules
    "typeRoots": ["./node_modules/@types", "./src/types"]
  }
}

# Check if axios bundles its own types
node -e "const p = require('axios/package.json'); console.log(p.types || p.typings)"
# Output: index.d.ts

# Check if lodash bundles types (it doesn't — use @types/lodash)
node -e "const p = require('lodash/package.json'); console.log(p.types)"
# Output: undefined

# You can also just try to install and see what the compiler says:
npm install lodash
# In your editor: import _ from 'lodash';
# TypeScript error: Could not find a declaration file for module 'lodash'.
# Try: npm i --save-dev @types/lodash

node_modules/@types/node/
  index.d.ts          ← main entry (re-exports all modules)
  fs.d.ts             ← types for the 'fs' module
  path.d.ts           ← types for the 'path' module
  http.d.ts           ← types for the 'http' module
  events.d.ts         ← types for the 'events' module
  package.json        ← points to index.d.ts
  ...

// When you write this in your code:
import { readFileSync } from 'fs';
import { join } from 'path';

// TypeScript resolves:
// node_modules/@types/node/fs.d.ts  → readFileSync type
// node_modules/@types/node/path.d.ts → join type

const content = readFileSync(join(__dirname, 'config.json'), 'utf-8');
//    ^? string (typed correctly thanks to @types/node)

# If you have express@4.18.x, install the 4.x types:
npm install --save-dev @types/express@^4

# If you have express@5.x, install the 5.x types:
npm install --save-dev @types/express@^5

// Good version alignment in package.json
{
  "dependencies": {
    "express": "^4.18.2",
    "lodash":  "^4.17.21"
  },
  "devDependencies": {
    "@types/express": "^4.17.21",
    "@types/lodash":  "^4.14.202",
    "@types/node":    "^20.0.0"
  }
}

/// <reference types="node" />
/// <reference types="jest" />

// Now __dirname and describe() are available in this file
// even if they aren't globally included by tsconfig.json

describe('file utilities', () => {
  it('resolves paths', () => {
    const p = require('path').join(__dirname, 'fixtures');
    expect(p).toContain('fixtures');
  });
});

# axios bundles its own types since v1.0
# If you also have @types/axios installed, remove it:
npm uninstall @types/axios

// Check which types file TypeScript is actually using:
// Hover over the import in your editor, or:
import axios from 'axios';
//     ^? — hover shows the resolved type file path

// Or check with tsc directly:
// npx tsc --traceResolution 2>&1 | grep axios

// src/types/lodash.d.ts
// Imagine @types/lodash is missing a function we need to type
import 'lodash';

declare module 'lodash' {
  interface LoDashStatic {
    // Add the missing method signature
    sumBy<T>(collection: T[], iteratee: (item: T) => number): number;
  }
}

// Now TypeScript accepts this without errors:
import _ from 'lodash';
const total = _.sumBy(orders, (o) => o.price);

types/
  lodash/
    index.d.ts         ← the declarations
    lodash-tests.ts    ← usage tests (must compile without errors)
    tsconfig.json      ← tsconfig for running the tests
    package.json       ← metadata (owners, dependencies)

npm install express
npm install --save-dev typescript @types/node @types/express ts-node

// tsconfig.json
{
  "compilerOptions": {
    "target": "ES2020",
    "module": "commonjs",
    "lib": ["ES2020"],
    "outDir": "dist",
    "strict": true,
    "esModuleInterop": true,
    "moduleResolution": "node"
  },
  "include": ["src/**/*"]
}

// src/app.ts
import express, { Request, Response, NextFunction } from 'express';

// All types come from @types/express
const app = express();

app.use(express.json());

// Route handler — req and res are fully typed
app.get('/users/:id', (req: Request, res: Response) => {
  const { id } = req.params; // string
  const { format } = req.query; // string | ParsedQs | string[] | ParsedQs[]

  res.json({ id, format });
});

// Error handler — four-parameter signature required by Express
app.use((err: Error, req: Request, res: Response, next: NextFunction) => {
  console.error(err.stack);
  res.status(500).json({ error: err.message });
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

// tsconfig.json
{
  "compilerOptions": {
    "skipLibCheck": true  // skip type-checking all .d.ts files
  }
}

// Common conflict example: @types/node vs browser globals
// @types/node declares 'global' as a namespace
// Browser code also uses 'global' patterns

// If you hit duplicate identifier errors from @types packages:
// Option 1 — skipLibCheck: true (broad)
// Option 2 — use the 'types' array to be selective:
// { "types": ["node"] }  // only include @types/node, exclude conflicting packages

# Install type-coverage
npm install --save-dev type-coverage

# Run — reports percentage of typed nodes
npx type-coverage
# Coverage: 95.23% (12034/12638)

# Show all any-typed locations
npx type-coverage --detail

# Fail CI if coverage drops below a threshold
npx type-coverage --atLeast 95