Can I use require and import in the same file?

Usually no. You either have a CJS file or an ESM file. In ESM, you can use `import` for CJS modules (mostly), but you cannot use `require` at all. In CJS, you can only import ESM modules using dynamic `await import()`, which is messy.

Why does Node.js have a thread pool if it's single-threaded?

The 'single thread' is just for your JavaScript execution (the Event Loop). To avoid blocking that loop during heavy I/O or crypto tasks, Node offloads that work to the Libuv thread pool in the background.

What happens if the thread pool is full?

Tasks will be queued and executed as soon as a thread becomes free. This can lead to performance bottlenecks in I/O-heavy applications if not monitored.

The Runtime: V8, Libuv, and the Module System

Node.js is not a programming language; it is a runtime environment. It takes the same JavaScript that runs in your browser and gives it access to your operating system's files, network, and memory. This is made possible by a sophisticated marriage between the V8 engine and the Libuv library. Understanding this relationship is key to debugging performance and architectural issues.

1. V8: The High-Speed Brain

Developed by Google for Chrome, V8 is the component that actually reads and executes your JavaScript code. It doesn't just "interpret" code line-by-line; it uses Just-In-Time (JIT) compilation to turn your JS into highly optimized machine code on the fly.

TurboFan: The optimizing compiler that analyzes your code while it runs and makes it faster based on usage patterns.
Hidden Classes: A trick V8 uses to speed up property access on objects by assuming they have a fixed "shape" internally.
Garbage Collection: V8 manages memory automatically, but poorly written code (like closures holding large objects) can still cause memory leaks.

2. Libuv: The Multitasking Muscle

While V8 handles the execution of JS, Libuv is the C++ library that handles everything else. It is the secret to Node's non-blocking I/O. When you read a file or make a network request, Node hands that task off to Libuv.

The Thread Pool: Libuv maintains a pool of worker threads (default is 4) to handle tasks that are too heavy for the single-threaded Event Loop, such as:

File System operations (fs)
DNS lookups
Cryptography (crypto)
Compression (zlib)

Pro-Tip: You can increase this pool size using process.env.UV_THREADPOOL_SIZE for specialized workloads.

3. The Great Module Divide: CommonJS vs ESM

Node.js is currently in a transition period between two module systems. This "Dual Module" world is one of the most common sources of frustration for developers today.

CommonJS (CJS)

const mod = require('./mod')
Synchronous loading
The original Node.js standard
Dynamic (can be inside if blocks)

ECMAScript Modules (ESM)

import mod from './mod.js'
Asynchronous loading
The browser/industry standard
Static (enables tree-shaking)

Which to use? Modern projects should prefer ESM (by setting "type": "module" in package.json). However, ESM requires explicit file extensions and handles __dirname differently, which often trips up developers.

4. The package.json "exports" Field

The modern way to define a library's interface is the exports field. It allows you to expose different files for CJS and ESM consumers, essentially future-proofing your package.

{
  "exports": {
    "import": "./dist/esm/index.js",
    "require": "./dist/cjs/index.js"
  }
}

Conclusion

Node.js is a powerhouse because it combines the extreme speed of V8 with the efficient multitasking of Libuv. By understanding how these layers interact—and how to navigate the shift from CommonJS to ESM—you can build tools and services that are both high-performing and modern. You're no longer just writing JS; you're orchestrating a runtime.