fs.readFile and the Poll Phase

·Hard·Node Event Loop

What runs when everything is scheduled from inside an I/O callback

Practice mode

Step

The main module runs

Synchronous work first: the require and this log.

Algorithm workspace

Execution

Step 1 / 16init

100%

node

timers

→pending

→poll (I/O)

→check

→close

↻

running: main script

Call stack

main module

Queues

process.nextTickmicrotask

drains first, and fully — even callbacks added while draining

empty

Microtasks (promises)microtask

drains after nextTick, between every phase

empty

Timers (setTimeout / setInterval)macrotask

timers phase — only callbacks whose delay has elapsed

empty

Poll (I/O callbacks)macrotask

poll phase — fs, net, dns completions

empty

Check (setImmediate)macrotask

check phase — runs immediately after poll

empty

Console output

1start

both microtask queues drain after every callback, not just between phases · a file read lands in poll

step 1 / 16

Space play← → stepHome reset

Time—·SpaceO(n) queued callbacks

Execution

1const fs = require('fs');
2 
3console.log('start');//output: 1
4 
5fs.readFile(__filename, 'utf8', (err, data) => {
6  console.log('file read');
7 
8  process.nextTick(() => console.log('nextTick in I/O'));
9  Promise.resolve().then(() => console.log('promise in I/O'));
10 
11  setTimeout(() => console.log('timeout in I/O'), 0);
12  setImmediate(() => console.log('immediate in I/O'));
13});
14 
15setTimeout(() => console.log('timer from main'), 0);
16 
17console.log('end');

output1

Command Palette

fs.readFile and the Poll Phase