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Node.js · interactive

The event loop, phase by phase

Synchronous code finishes first. Queued callbacks then run in a fixed order. Turn some off and run the loop again.

Call stack

empty

nextTick queue

process.nextTick

Promise queue

Promise.then

Timers

setTimeout(0)

I/O (poll)

fs.readFile callback

Check

setImmediate

Run one turn and watch callbacks leave their queues for the call stack.

Queue callbacks, then run one turn.

  1. 1

    The call stack runs to the end. Nothing queued can start yet.

    console.log("start")
    console.log("end")

  2. 2

  3. 3

  4. 4

  5. 5

  6. 6

This is the order when the timer is already due and the file callback is already ready. If the timer is still waiting, poll can run first.

In production

Tools and technologies

One thread interleaves work with I/O callbacks; similar patterns appear outside Node too.

Not tied to one language — use these names when you search docs or design a system.