How Node.js works
Node.js uses one main thread plus a helper system so it can wait for slow things like files and networks without freezing.
In this page:
How Node.js works
Node.js runs your JavaScript on a single thread, but hands slow I/O work (files, network, timers) to the operating system and a thread pool via libuv.
When the work finishes, a callback is queued and run on the main thread. This non-blocking model lets one process serve thousands of connections.
Note:
CPU-heavy work blocks the single thread; move it to worker threads or another process.
Example: How Node.js works
console.log("1. start");
setTimeout(() => console.log("3. timer callback (after I/O style wait)"), 0);
console.log("2. end of script");
// Output:
// 1. start
// 2. end of script
// 3. timer callback (after I/O style wait)⚠️ Run this in your own terminal or Node.js environment.
Related Topics
{# common_mistakes/chapter_summary/browser_support: on Hindi pages the
view already swaps in the hi_ translation fields (or blanks these
out if untranslated), so this renders correctly for both languages
without a lang_code check here. #}
Common Mistakes
- Running long CPU loops on the main thread
- Assuming Node uses one thread for everything including I/O
- Blocking the event loop with synchronous file APIs in servers
Chapter Summary
- JavaScript runs on one main thread
- libuv handles async I/O
- Callbacks run when work completes
- Avoid blocking the main thread
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