Event Loop Highlights Summary

Every highlight bullet from the 9 pages in this section, gathered on one page and grouped by the page it came from.

The Node.js Event Loop

• The event loop is a fixed cycle of phases libuv runs for as long as the process has pending work
• JavaScript runs to completion - the loop only advances once the call stack is empty
• Microtasks drain completely between callbacks, outside the phase cycle itself
• The loop is cooperative, not preemptive - one uncooperative callback stalls the whole cycle

Event Loop Basics

• One JavaScript thread per process handles all callbacks
• I/O completion schedules callbacks on the event loop
• Microtasks run before the next macrotask phase
• Blocking the loop delays every concurrent client

libuv Phases

• Each loop iteration runs phases in a fixed order
• Timers phase fires setTimeout and setInterval callbacks
• Poll phase waits for I/O and executes read/write callbacks
• Check phase runs setImmediate callbacks

Microtasks vs Macrotasks

• Microtasks include Promises and queueMicrotask
• process.nextTick runs before Promise microtasks
• Macrotasks include timers and setImmediate
• Microtasks drain completely before the next macrotask

async/await in Node

• async/await is syntactic sugar over Promises
• try/catch preserves stack traces for awaited failures
• Parallel work uses Promise.all, not sequential await
• Top-level await works in ESM modules

Callbacks vs Promises Migration

• fs/promises replaces most fs callback APIs
• util.promisify wraps callback-last functions
• Migrate leaf utilities before route handlers
• Preserve error-first callback semantics in wrappers

Timers & Scheduling

• Timers are minimum delays, not real-time guarantees
• setInterval can overlap if callbacks run longer than the interval
• timers/promises and AbortSignal support cancellable delays
• Use monotonic clocks for deadlines, timers for deferral

Detecting Event-Loop Blockage

• monitorEventLoopDelay measures lag between scheduled work
• eventLoopUtilization shows how saturated the loop is
• blocked-at attributes sync blocks to stack traces
• Alert on p99 loop delay, not just CPU percentage

Event Loop Best Practices

• Keep request handlers non-blocking and thin
• Parallelize independent I/O with Promise.all
• Never recurse process.nextTick or flood microtasks
• Measure loop lag, not just CPU and memory