achou
demetris
Another thing to watch for is whether you actually need debouncing.
For example, debouncing is often recommended for handlers of the resize event, but, in most cases, it is not needed for handlers of observations coming from ResizeObserver.
I think this is the case for other modern APIs as well. I know that, for example, you don’t need debouncing for the relatively new scrollend event (it does the debouncing on its own).
yurishimo
Sad that the `scrollend` event isn't supported by Safari and doesn't look to be a part of their release this fall either.
demetris
Yep. Apple is a weird organization. scrollend is in Chrome since May 2023 and in Firefox since January 2023.
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joeframbach
That doesn't sound correct. An async function ought to return a _new_ Promise on each invocation, and each of those returned Promises are independent. Are you conflating memoization? Memoized functions will have these problems with denouncing, but not your standard async function.
ricardobeat
If I understand it correctly, they're saying the debounce function itself usually implements memoization in a way that will return you stale promises.
cnity
Reactive programming (such as with RxJS [0]) can be a good solution to this, since they provide primitives that understand time-based dependencies.
ffsm8
Debouncing correctly is still super hard, even with rxjs.
There are always countless edge cases that behave incorrectly - it might not be important and can be ignored, but while the general idea of debouncing sounds easy - and adding it to an rxjs observable is indeed straightforward...
Actually getting the desired behavior done via rxjs gets complicated super fast if you're required to be correct/spec compliant
ncr100
So - events / handlers may need to be tagged as "for human interaction"? (to avoid over-filtering signals?)
pas
this sounds interesting but it's a bit too early here for me. by any chance can we (not simply a royal we :D) ask you to provide a code example (of a correct implementation), or a link to one? many thanks!
tossandthrow
Damn, there was another thread not too long ago claiming that a sync does not mean concurrent - this would have been a great example to bring up.
ignoramous
Such stuff has first-class support in Kotlin: Structured concurrency simplifies multi-threaded programming pretty effectively.
kazinator
Debouncing refers to cleaning up the signal from an opening and closing switch contact so that the cleaned signal matches the intended semantics of the switch action (e.g. one simple press of a button, not fifty pulses).
The analogy here is poor; reducing thrashing in those obnoxious search completion interfaces isn't like debouncing.
Sure, if we ignore everything about it that is not like debouncing, and we still have something left after that, then whatever is left is like debouncing.
One important difference is that if you have unlimited amounts of low latency and processing power, you can do a full search for each keystroke, filter it down to half a dozen results and display the completions. In other words, the more power you have, the less important it is to do any "debouncing".
Switch debouncing is not like this. The faster is your processor at sampling the switch, the more bounces it sees and consequently the more crap it has to clean up. Debouncing certainly does not go away with a faster microcontroller.
maxbond
It's the term used in frontend dev. It is actually a little worse than you're imagining, because we're not sampling, we're receiving callbacks (so more analogous to interrupts than sampling in a loop). Eg the oninput callback. I've used it for implementing auto save without making a localStorage call on every key press, for example.
I think it makes sense if you view it from a control theory perspective rather than an embedded perspective. The mechanics of the UI (be that a physical button or text input) create a flaggy signal. Naively updating the UI on that signal would create jank. So we apply some hysteresis to obtain a clean signal. In the day way that acting 50 times on a single button press is incorrect behavior, saving (or searching or what have you) 50 times from typing a single sentence isn't correct (or at least undesired).
The example of 10ms is way too low though, anything less than 250ms seems needlessly aggressive to me. 250ms is still going to feel very snappy. I think if you're typing at 40-50wpm you'll probably have an interval of 100-150ms between characters, so 10ms is hardly debouncing anything.
zeta0134
Additionally, regardless of naming, debouncing is an accessibility feature for a surprisingly large portion of the population. Many users who grew up with double-click continue to attempt to provide this input on web forms, because it mostly works. Many more with motor control issues may struggle to issue just a single click reliably, especially on a touchscreen.
gibsonsmog
Holy moly, for years I've had in the back of my head this thought about why, earlier in my career, I'd see random doubly submitted form submissions on certain projects. Same form code and processing as other sites, legitimate submissions too. Eventually we added more spam filtering and restrictions unrelated to these legitimate ones, but it was probably the double-click users causing those errant submissions. I'd never even have thought of those users. Fascinating
account42
> 250ms is still going to feel very snappy
WTF no it won't.
schwartzworld
For the kind of behaviors they are describing it would. An extra 250ms waiting for an app to load is a lot, but for something like the described autosave behavior, waiting for a 250ms pause in typing before autosaving or making a fetch call is pretty snappy.
Findecanor
An office keyboard's own debouncing could delay a key press 30 ms, and then the OS, software and graphics/monitor hardware would delay it just as much before the user could see the character on screen. So, indeed, 10 ms is much too low.
maxbond
Perhaps the people at MDN are 10x typists, with competition-grade gaming keyboards.
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Findecanor
I agree that this is a bad analogy.
I've programmed my own keyboards, mice and game controllers. If you want the fastest response time then you'd make debouncing be asymmetric: report press ("Make") on the first leading edge, and don't report release ("Break") until the signal has been stable for n ms after a trailing edge. That is the opposite of what's done in the blog article.
Having a delay on the leading edge is for electrically noisy environments, such as among electric motors and a long wire from the switch to the MCU, where you could potentially get spurious signals that are not from a key press. Debouncing could also be done in hardware without delay, if you have a three-pole switch and an electronic latch.
A better analogy would perhaps be "Event Compression": coalescing multiple consecutive events into one, used when producer and consumer are asynchronous. Better but not perfect.
Sharlin
Debouncing is established terminology in UI and other event-handling stuff at this point, and has been for a decade. It's a bit too late to complain. Language evolves and not all new uses of words are good analogies.
account42
It's also worth mentioning that real debouncing doesn't always have to depend on time when you have an analog signal. Instead you could have different thresholds for going from stat A to B vs going from B to A with enough distance between those threshold that you won't switch back and forth during an event. This can even be implemented physically in the switch itself by having separate ON and OFF contacts.
haileys
Debouncing is a term of art in UI development and has been for a long time. It is analogous to, but of course not exactly the same as, debouncing in electronics.
kube-system
It's a word borrowed for a similar concept. This is so common in software, it is basically the norm. There are hundreds of analogistic terms in software.
Tade0
> One important difference is that if you have unlimited amounts of low latency and processing power, you can do a full search for each keystroke,
But you don't want that, as it's useless. Until the user actually finished typing, they're going to have more results than they can meaningfully use - especially that the majority will be irrelevant and just get in the way of real results.
The signal in between is actually, really not useful - at least not on first try when the user is not aware what's in the data source and how can they hack the search query to get their results with minimal input.
maxbond
No one wants to see results for the letter "a", no one wants their database processing that search, and updating the UI while you're typing can be really distracting.
meindnoch
>No one wants to see results for the letter "a"
Don't make assumptions about what the user may or may not want to search for.
E.g. in my music collection I have albums from both !!! [1] and Ø [2]. I've encountered software that "helpfully" prevented me from searching for these artists, because the developers thought that surely noone would search for such terms.
_______
[1] https://www.discogs.com/artist/207714-!!! ← See? The HN link highlighter also thinks that URLs cannot end with !!!.
extra88
As a user, I often do want a list to start from a single letter. In a browser address bar, it could start showing items Amazon, Apple, etc.
davnicwil
Thank you for this comment! Suddenly 'bouncing' makes total sense as a mental image when before it only vaguely tracked in some abstract way about tons of tiny events bouncing around and triggering things excitedly until you contain them with debounce() :-)
Come to think of it throttle is the much easier to understand analogy.
Izkata
Throttling is a different thing though. Debouncing is waiting until the input has stopped occurring so it can run on the final result, throttling is running immediately on the first input and blocking further input for a short duration.
jiehong
In electronics, I think we'd use a latch, so it switches high, and stays high despite input change.
Doesn't really apply to a search box, where it's more of a delayed event if no event during a specific time window, only keeping last event.
naasking
> In electronics, I think we'd use a latch, so it switches high, and stays high despite input change.
RC circuits are more typical, you want to filter out high frequency pulses (indicative of bouncing) and only keep the settled/steady state signal. A latch would be too eager I think.
thomascountz
I recently wrote a bit about debouncing fetch using timeouts and AbortController, including small demos, here: https://thomascountz.com/2025/07/02/debouncing-api-calls
ncann
I needed an implementation of debounce in Java recently and was surprised to find out that there's no existing decent solution - there's none from the standard library, nor popular utilities libraries like Guava or Apache Commons. There are some implementations floating around like on Stackoverflow but I found them lacking, either there's no thread safety or there's no flexibility in supporting the execution of the task at the leading edge or trailing edge or both. Anyone has a good recommendation on a good implementation?
_def
I think it's a fitting analogy. Depends on the intended behaviour, really.
That said, this is a good resource on the original meaning: https://www.ganssle.com/debouncing.htm
lma21
I've always used the term Coalescing in the past: https://en.wikipedia.org/wiki/Coalescing_(computer_science)
joeframbach
And I've used coalesce to describe Array.prototype.reduce and Object.assign as well.
game_the0ry
Not sure if anyone else has noticed, but this has been a super popular interview question for front end interviews.
G_o_D
Debounce -> Like when we throw a ball once on ground, but it keeps bouncing, To prevent that
Human interaction with circuits, sensors, receptors, occur like that
When we click a keyboard key or switch circuit switch the receptors are very sensitive
we feel we did once but during that one press our fingers hands are vibrating multiple times hence the event get registered multiple times due to pulsating, hence all after first event, the second useful event that can be considered legitimate if the idle period between both matches desired debounce delay
in terms of web and software programming or network request handling
it is used as a term to debounce to push away someone something aggresive
Example wise
a gate and a queue Throttling -> gate get opened every 5 min and let one person in, no matter what
Debounce -> if the persons in queue are deliberately being aggressive thrashing at door to make it open we push them away Now instead of 5 min, we tell them you have to wait another 5 min since you are harassing, if before that they try again, we again tell them to wait another 5 min Thus debounce is to prevent aggresive behaviour
In terms of say client server request over network
We can throttle requests processed by server, let say server will only process requests that happen every 5 min like how apis have limit, during that before 5min no matter how many request made they will be ignored
But if client is aggressive like they keep clicking submit button, keep making 100s of requests that even after throttling server would suffer kind of ddos
so at client side we add debounce to button click event
so even if user keep clicking it being impatient, unnecessary network request will not be made to server unless user stop
null
One thing to watch out for when using debounce/throttle is the poor interaction with async functions. Debounced/throttled async functions can easily lead to unexpected behavior because they typically return the last result they have when the function is called, which would be a previous Promise for an async function. You can get a result that appears to violate causality, because the result of the promise returned by the debounce/throttle will (in a typical implementation) be from a prior invocation that happened before your debounce/throttle call.
There are async-safe variants but the typical lodash-style implementations are not. If you want the semantics of "return a promise when the function is actually invoked and resolve it when the underlying async function resolves", you'll have to carefully vet if the implementation actually does that.