The answer
setTimeout runs in the timers phase, scheduled for after a delay. setImmediate runs in the check phase, right after the poll phase. On the next event loop tick, setImmediate runs before setTimeout if there are no pending timers.
libuv and Async I/O Interview Questions for Node.js
What is the difference between setTimeout and setImmediate?
More FAQs in libuv and Async I/O Interview Questions for Node.js
A C library that provides cross-platform async I/O for Node.js. It handles the event loop, file system and network operations, the thread pool for blocking work, and timers. Without libuv, Node.js would not be non-blocking.
Blocking code stops the main thread until it completes. Non-blocking code returns immediately and processes the result via a callback when complete. Node.js is single-threaded, so blocking code stops all requests, which is the biggest performance concern.
libuv has a thread pool (default 4 threads) for operations that cannot be done asynchronously, like certain file system, DNS, and crypto operations. These run off the main thread. Callbacks run on the main thread via the event loop.
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