What is npm? Explain the purpose of package.json and the difference between dependencies and devDependencies.

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Accepted Answer

## What is npm?

npm (Node Package Manager) is the **default package manager for Node.js**. It allows you to:

- Install third-party packages
- Share your own packages
- Manage project dependencies
- Run scripts

```bash
# Install a package
npm install express

# Install globally (CLI tools)
npm install -g nodemon

# Install as dev dependency
npm install --save-dev jest
```

## package.json

The `package.json` file is the **manifest of your project**. It contains metadata and configuration.

```json
{
  "name": "my-app",
  "version": "1.0.0",
  "description": "A sample Node.js app",
  "main": "index.js",
  "scripts": {
    "start": "node index.js",
    "dev": "nodemon index.js",
    "test": "jest"
  },
  "dependencies": {
    "express": "^4.18.2",
    "mongoose": "^7.0.0"
  },
  "devDependencies": {
    "jest": "^29.0.0",
    "nodemon": "^3.0.0"
  }
}
```

## dependencies vs devDependencies

| Type | Purpose | Installed in Production? |
|------|---------|-------------------------|
| **dependencies** | Required to run the app | Yes |
| **devDependencies** | Only for development | No |

**dependencies** (runtime):
- Express, Mongoose, Axios
- Libraries your app needs to function

**devDependencies** (development only):
- Jest, Mocha (testing)
- Nodemon (auto-restart)
- ESLint, Prettier (code quality)
- TypeScript, Babel (build tools)

## Key Commands

```bash
# Create package.json
npm init

# Install all dependencies
npm install

# Install production only
npm install --production

# Update packages
npm update

# Check outdated
npm outdated
```

## package-lock.json

Locks exact versions of all dependencies for reproducible builds. **Always commit this file** to version control.