What is the difference between synchronous and asynchronous code in Node.js, and how does the Event Loop manage them?
Quick Revision Summary
Node.js utilizes a single-threaded Call Stack with libuv. The Event Loop processes non-blocking I/O callbacks, prioritizing microtasks (Promises) over macrotasks (Timers).
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Model Answer & Deep-Dive Explanation
Node.js runs on a single-threaded JavaScript execution engine (V8) paired with libuv to handle asynchronous non-blocking I/O.
- Synchronous Code: Executes line-by-line on the Call Stack. Heavy synchronous operations (e.g.
fs.readFileSync, CPU loops) block the entire thread, halting all incoming user requests. - Asynchronous Code: Delegated to libuv's thread pool or the operating system ke el (for network sockets). When the async task completes, its callback is queued into the Event Loop.
The Event Loop cycles through distinct phases: Timers (setTimeout) -> Pending Callbacks -> Idle/Prepare -> Poll (I/O) -> Check (setImmediate) -> Close Callbacks. Microtasks (process.nextTick and resolved Promises) run immediately after the current operation finishes, before the next Event Loop phase.
Interviewer Evaluation Points & Scoring Rubric
- 1 1. Mentions single-threaded V8 and libuv. 2. Explains non-blocking async delegation. 3. Correctly predicts microtask vs macrotask execution order.
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