CODEWITHSUNDEEP
javascriptnode.jssettimeoutevent-loop
Ivan Velichko
Ivan Velichko

Reputation: 6709

setTimeout() triggers a little bit earlier than expected

UPD: The question What is the reason JavaScript setTimeout is so inaccurate? asks why the timers in JavaScript are inaccurate in general and all mentions of the inaccuracy are about invocations slightly after the specified delay. Here I'm asking why NodeJS tolerates also invocations even before the delay? Isn't it an error-prone design of the timers?

Just found an unexpected (to me only?) behaviour of NodeJS setTimeout(). Some times it triggers earlier than the specified delay. 

function main() {
  let count = 100;
  while (count--) {
    const start = process.hrtime();
    const delay = Math.floor(Math.random() * 1000);

    setTimeout(() => {
      const end = process.hrtime(start);
      const elapsed = (end[0] * 1000 + end[1]/1e6);
      const dt = elapsed - delay;
      if (dt < 0) {
        console.log('triggered before delay', delay, dt);
      }
    }, delay);
  }
}

main();

On my laptop output is:

$ node --version
$ v8.7.0

$ node test.js
triggered before delay 73 -0.156439000000006
triggered before delay 364 -0.028260999999986325
triggered before delay 408 -0.1185689999999795
triggered before delay 598 -0.19596799999999348
triggered before delay 750 -0.351709000000028

Is it a "feature" of the event loop? I always thought that it must be triggered at least after delay ms.

Upvotes: 3

Views: 1296

Answers (2)

a.v
a.v

Reputation: 1

From node timers doc, about settimeout:

The only guarantee is that the timeout will not execute sooner than the declared timeout interval

Upvotes: -1

Dave Newton
Dave Newton

Reputation: 160170

From the NodeJS docs:

The callback will likely not be invoked in precisely delay milliseconds. Node.js makes no guarantees about the exact timing of when callbacks will fire, nor of their ordering. The callback will be called as close as possible to the time specified.

As you increase the number of intervals (you have 100) the accuracy decreases, e.g., with 1000 intervals accuracy is even worse. With ten it's much better. As NodeJS has to track more intervals its accuracy will decrease.

We can posit the algorithm has a "reasonable delta" that determines final accuracy, and it does not include checking to make sure it's after the specified interval. That said, it's easy enough to find out with some digging in the source.

See also How is setTimeout implemented in node.js, which includes more details, and preliminary source investigation seems to confirm both this, and the above.

Upvotes: 3

Related Questions