Make "scrollLeft" / "scrollTop" changes not trigger scroll event listener

Question

Currently my program is in a spot where it both listens for the user to scroll a certain element, but also, at times, automatically scrolls this element by itself. (Not a gradual, pretty scroll, but an instant jump. It makes sense in context, I swear.)

Is there a way to make the scroll event not trigger if the scrolling was done by setting scrollLeft or scrollTop? My first thought was a basic switch, like:

ignoreScrollEvents = true;
element.scrollLeft = x;
ignoreScrollEvents = false;

function onScroll() {
  if(ignoreScrollEvents) return false;
}

but since events don't trigger immediately (oops, duhh), that's not a workable solution. What other sort of answers could I try? I'm also using jQuery, if that helps anything.

Answer 1

Easiest generic method? Just reset the flag in the event handler. You'll want to check first if you're actually changing the value, as otherwise an event won't be fired - as Rodrigo notes, a good practice here is to factor this out into so-called "setter" functions:

function setScrollLeft(x) {
  if (element.scrollLeft != x) {
    ignoreScrollEvents = true;
    element.scrollLeft = x;
  }
} 

...

function onScroll() {
  const ignore = ignoreScrollEvents;
  ignoreScrollEvents = false;
  
  if (ignore) {
    return false;
  }

  ...
}

But, depending on your needs, you may already be storing the scroll position somewhere; if so, just update and check that as your flag. Something like:

element.scrollLeft = x;
currentScrollLeft = x;

...

function onScroll() {
  // retrieve element, etc... 
  if (element.scrollLeft == currentScrollLeft) {
    return false;
  }

  ...
}