What are V8 and libuv, and what role does each play in Node.js?

V8 runs your JavaScript, and libuv gives Node its event loop and async I/O.

V8 is Google's JavaScript engine. It does these jobs:

• It parses and compiles your JavaScript.
• It runs the code with an interpreter and optimizing JIT compilers. A JIT (just-in-time) compiler turns code into fast machine code while the program runs.
• It manages the heap (the memory for objects) and garbage collection.

V8 knows nothing about files, sockets or timers.

libuv is a C library. It provides the event loop and asynchronous I/O that works on every platform:

• TCP and UDP sockets
• timers
• child processes
• signals
• the thread pool for file system work and other blocking work

Node connects the two with C++ bindings. A binding is C++ code that JavaScript can call. Here is what happens when you call fs.readFile:

• The JavaScript API in Node's core library calls a binding.
• The binding sends the work to libuv.
• libuv reports that the work is done.
• Node calls your JavaScript callback through V8.

Node also includes other libraries, such as OpenSSL for crypto and llhttp for HTTP parsing.