setInterval, setTimeout and the Timers Phase

·Medium·Node Event Loop

How an interval re-arms, and where a one-shot timer cuts in

Practice mode

Step

The counter starts at zero

n lives in the module scope, so every run of the interval callback reads and writes the same variable.

Algorithm workspace

Execution

Step 1 / 14init

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

Console output

no output yet

the timers phase runs every callback whose deadline has passed · an interval re-arms itself each time it runs

step 1 / 14

Space play← → stepHome reset

Time—·SpaceO(1) — one heap entry per timer

Execution

1let n = 0;//n: 0
2 
3const id = setInterval(() => {
4  n += 1;
5  console.log('interval', n);
6 
7  if (n === 3) {
8    clearInterval(id);
9    console.log('cleared');
10  }
11}, 10);
12 
13setTimeout(() => console.log('timeout at 25ms'), 25);
14 
15console.log('sync done');

Command Palette

setInterval, setTimeout and the Timers Phase