setImmediate vs setTimeout(fn, 0)

·Medium·Node Event Loop

Nondeterministic at the top level, guaranteed inside an I/O callback

Practice mode

Step

The main module runs

Synchronous work first, as always — the event loop has not started yet.

Algorithm workspace

Top level (order not guaranteed)

Step 1 / 9init

100%

node

timers

→pending

→poll (I/O)

→check

→close

↻

running: main script

Call stack

main module

Queues

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

1main start

phase order is timers → pending → poll → check → close · setImmediate is the check phase, setTimeout is the timers phase

step 1 / 9

Space play← → stepHome reset

Top level (order not guaranteed)

1console.log('main start');//output: 1
2 
3setTimeout(() => console.log('setTimeout 0'), 0);
4setImmediate(() => console.log('setImmediate'));
5 
6console.log('main end');

output1

Command Palette

setImmediate vs setTimeout(fn, 0)