libuv and Async I/O Interview Questions for Node.js
libuv and async I/O come up in Node.js interviews. Here are the common questions and how to answer them.
libuv and Async I/O Interview Questions for Node.js
libuv and async I/O come up in Node.js interviews to test depth. Here are the common questions.
What is libuv and what does it do in Node.js?
libuv is 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, and timers. Without libuv, Node.js would not be non-blocking.
What is the difference between blocking and non-blocking code?
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.
How does the thread pool work in Node.js?
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.
What are the event loop phases?
Timers (setTimeout), pending callbacks, idle/prepare (internal), poll (I/O callbacks), check (setImmediate), and close callbacks. Each phase runs specific callbacks in order, which explains the order async code runs.
What is the difference between setTimeout and setImmediate?
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.
The Takeaway
Know what libuv is and does, blocking vs non-blocking, how the thread pool works, event loop phases, and setTimeout vs setImmediate. These test real understanding of Node.js internals.
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.
Timers (setTimeout), pending callbacks, idle/prepare (internal), poll (I/O callbacks), check (setImmediate), and close callbacks. Each phase runs specific callbacks in order, which explains the order async code runs.
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.
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