The setTimeout function does not preserve FIFO ordering for equal delays #447
Description
When scheduling multiple timer callbacks with the same delay using worker-timers’ setTimeout, callbacks may fire out of order. This violates the HTML Standard’s requirement that timer callbacks enqueued on the same task source must execute in FIFO order when their delays expire simultaneously.
Steps to Reproduce
Schedule multiple callbacks synchronously with the same delay (e.g. 10 ms):
import { setTimeout as workerSetTimeout } from 'worker-timers';
(async () => {
const results = [];
await new Promise((resolve) => {
for (let i = 1; i <= 10; i++) {
workerSetTimeout(() => {
results.push(i);
if (results.length === 10) {
resolve();
}
}, 10);
}
});
alert(`Results: ${results.join(', ')}`);
})();Expected result
Alert with (1, 2, 3, ...)
Actual result
Alert with numbers in quite a random order.
ℹ️ The issue can also be reproduced in Karma test like:
import { setTimeout as workerSetTimeout } from 'worker-timers';
describe('setTimeout order', () => {
it('should resolve consequent setTimeout calls with the same delay in order 1', async () => {
const results: number[] = [];
await new Promise<void>((resolve) => {
for (let i = 1; i <= 10; i++) {
workerSetTimeout(() => {
results.push(i);
if (results.length === 10) {
resolve();
}
}, 10);
}
});
expect(results).toEqual([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
});
});