Promise vs setTimeout — interactive event loop example

1console.log('start');
2 
3setTimeout(() => {
4  console.log('timeout');
5}, 0);
6 
7Promise.resolve().then(() => {
8  console.log('promise');
9});
10 
11console.log('end');

The loop always drains microtasks before touching the task queue. The promise callback — queued AFTER the timer fired — still goes first.

6 / 9

Call stack

promise cb

Web APIs — browser land

The event loop — is the call stack empty? → drain the microtask queue first → then take one callback.

Microtask queue — priority

Callback queue

timeout cb

Console

01start

02end

Fig. — Promise vs setTimeout · step 6 of 9

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Promise vs setTimeout — interactive event loop example

Call stack ?

Web APIs — browser land ?

Microtask queue — priority ?

Callback queue ?

Console ?

Call stack

Web APIs — browser land

Microtask queue — priority

Callback queue

Console