- Even without this performance hit, an often used way for implementing 'auto scroll to top/bottom' is to first check if there's no other stuff coming in before starting to actually scroll. This goes unnoticed by the user but drastically reduces the number of updates (and in this case, number of calls to scrollHeight) needed when a lot of data is coming in in batches. Principle is like: receive message, add to buffer and start timer of like 50ms. Upon timer tick copy everything from buffer (which in the meantime can have accumulated more data) to rendering and only then update scroll.
- Yup. Anything layout-related could be costly to read, and could force a layout to occur if modified since the last read (even within the same synchronous javascript code). Most things are not deferred until the next layout pass, which is one of the reasons virtual DOM got popular: it batches changes for you.
by francisofascii
3 subcomments
- If scrollheight is not a performant property, than it shouldn't be a property. It should have been a method called calculateScrollHeight() or something to indicate that it is not cheap.
- Properties are to allow dynamic actions for what appear to be simple variable accesses. They don't magically make those as fast as accessing a variable; they are a syntactic convenience to allow assignment and using the value implicitly rather than having to invoke a function/method. They could have cached the value and kept a dirty flag, but then everything that affected the value would have to be sure to mark it as dirty or result in subtle bugs.
- The biggest performance bomb you can have in your code is a loop that does something like
for (...) {
el.style.height = `${something}px`;
whatever.value = el.style.offsetHeight;
}
This forces the browser to recalculate layout multiple times in a single frame. Separating layout changing code from measurement code will help a lot here (most frameworks out there have solved this so we don't have to be too concerned about it though).
- I had a severe performance issue with streaming messages in a custom chatbot UI. I was able to resolve by enqueuing the streamed token chunks and processing them in batches at requestAnimationFrame boundaries. Forcing an immediate repaint on every received chunk seems to break down in a non-linear way. For small conversations with few messages you almost can't tell the difference.
- > I just assumed that readonly properties will always be pretty performant in general.
This is also the case for variables. var is faster than const, because for const (and let) the engine must do additional work (related to enforcing block-level scope, if I am not mistaken).
by ricardobeat
0 subcomment
- Layout-triggering properties used to be common knowledge for frontend development. Now you just try the next npm library that promises to be a little faster…
by rootlocus
1 subcomments
- Which is why Zig's top feature on its webpage is "No hidden control flow."
- Sometimes I think about this: using readonly is generally recommended at the architectural level. But it's technically difficult to choose the right trade-off when you need to remove that abstraction. The reason readonly is recommended is because it's a form of encapsulation, and using it means you're promising immutability for that state.
Sometimes you break down abstractions to reduce actual costs, but that can end up being bad for the overall structure. So you say you're breaking it down because the cost ruined the perceived performance, but it's always difficult to decide whether it's better to keep the overall beauty or to break the abstraction locally.
Speed and user UX are important, but if it's a screen the user is constantly watching, you might remove the abstraction. However, if it's something like a waiting screen after payment, you'd probably keep it. In the end, what matters is the user flow
- > readonly property
Um, what readonly property?
by jauntywundrkind
1 subcomments
- Afaik let in JavaScript also has a sizable penalty too, if not quite as bad as const. Var on and on.
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