Node.js Event Loop Visualizer

Interactive visualization of Node.js asynchronous execution model

Examples

Understanding execution order

Code Editor

Call Stack (Synchronous Code)

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process.nextTick()

Highest priority - runs before microtasks

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Microtask Queue

Promises - runs after nextTick

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Event Loop Phases (libuv)

1. Timers

setTimeout, setInterval callbacks

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2. Pending I/O

I/O callbacks deferred to next iteration

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3. Poll

Retrieve new I/O events, execute callbacks

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4. Check

setImmediate callbacks

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5. Close

Close event callbacks (socket.on("close"))

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Console Output

Waiting for execution...

Execution Order

  1. 1Synchronous code (Call Stack)
  2. 2process.nextTick() callbacks
  3. 3Promise microtasks
  4. 4Event Loop phases (Timers → Pending → Poll → Check → Close)

Key Differences from Browser

  • ✓process.nextTick() - Node.js only, executes before any I/O
  • ✓setImmediate() - Node.js only, runs after I/O events
  • ✓Six distinct phases vs browser's simpler model
  • ✓Built on libuv for cross-platform async I/O