Nodejs Runtime Practice Exam

Node.js is a JavaScript runtime built on Chrome's V8 engine. It allows running JavaScript outside the browser, using an event-driven, non-blocking I/O model that makes it lightweight and efficient for building scalable network applications.

The event loop is the mechanism that allows Node.js to perform non-blocking I/O operations despite JavaScript being single-threaded. It continuously checks the call stack and callback queue, executing callbacks when the stack is empty. Phases include: timers → pending callbacks → idle/prepare → poll → check → close callbacks.

• Timers – executes setTimeout and setInterval callbacks
• Pending callbacks – I/O callbacks deferred to next iteration
• Poll – retrieves new I/O events
• Check – executes setImmediate callbacks
• Close callbacks – e.g. socket.on('close')

setImmediate() executes in the check phase of the event loop, after the poll phase completes. setTimeout(fn, 0) executes in the timers phase on the next iteration. When called within an I/O callback, setImmediate() always fires before setTimeout().

process.nextTick() fires immediately after the current operation, before the event loop continues. It is processed on the microtask queue, not part of any event loop phase. setImmediate() fires on the next iteration of the event loop. Overusing nextTick can starve I/O.