Vladimir Klepov as a Coder

Two practical uses for capture event listeners

Normally, JS event are handled while bubbling up the DOM tree, and we’ve all had the pleasure to catch an event from a child node on its parent. You’d even be excused for thinking that’s the only way DOM events move. Many also know there’s something else — events start at the document root, then go down to the affected element in a phase called “capture”, and only then “bubble” back up (not all do — more on this in a minute). See MDN article on events for more details on this mechanism.

I always saw this capture / bubble thing as a trick interview question similar to “what is the value of i++ + ++i“, not something useful in normal life. Yet, I have found several good uses for this knowledge nugget, and now I want to pass it over to you.

Observe non-bubbling events

It turns out that some events don’t bubble at all. I first encountered this when working on a zoom-to-fit feature in Yandex.mail. When an image is loaded, the email DOM box size may change, and you need to resize it a bit more. But to do this, you need to know when an image loads, and load is one naughty event that does not bubble. The original code was therefore a wild mess of

querySelectorAll('img').forEach(img => img.addEventListener('load', resize))

As far as I’m concerned, though, all events go through the capture phase with no exceptions, so I happily replaced it with

<div onLoadCapture={resize}>{email}</div>

See this in action in a sandbox I made.

I’m not sure the exact choice of non-bubbling events makes any sense. load and error do not bubble, so my first guess was “of course, it’s an image that loads, not the div itself”. However, input event bubbles, and by the same logic input happened in the input, not in the containing div. Your best bet is to consult the MDN.

Selectively prevent events inside a container

Another practical use of capture is when you don’t want the content of a container to respond to some user interactions. Capture handler on the container fires before any handlers on the content, so you get a chance to stopPropagation, and the listeners attached to the inner elements will never know something happened. I have used this on two occassions.

By coincidence, just today I used a capture listener to stop click event from firing on content during custom gesture detection. Touch devices nicely do it natively — if you happen to hit a button when scrolling a page, the button will not be clicked when you release a finger. We have a Gallery / Slider component that supports mouse drag, and firing click when switching slides might be unexpected. Fixed with a capture click listener.

Another case was making a react-based form readonly during synchronization with backend using change capture — see sandbox. I’ll be the first one to admit this was not the cleanest approach, but when choosing between this and reimplementing all the form controls to support disabling via context for a one-off feature, I think I made the right call.

How to attach a capture event handler

If you’ve ever messed with the trird argument of addEventListener, you know it comes with two signatures: the legacy addEventListener(evt, cb, true) where true is the boolean capture argument, and the more modern addEventListener(evt, cb, { capture: true }) designed to support the other flag, passive. Contrary to the popular belief, if you just want capture, you don’t need complicated feature detection — using the boolean parameter is fine and safe, since no backwards compatibility was ever broken here. So, to add a capture event listener using the DOM API:

const onClick = e => console.log(e);
element.addEventListener('click', onClick, true);
// NOTE: removing a capture listener requires capture flag, too
element.removeEventListener('click', onClick, true);

In React, you just <div onClickCapture={onClick}> — all the events support this except onMouseEnter / Leave, because React is trying to be smart. I’m no expert on other frameworks, but I’m hearing Vue has <div v-on:click.capture="onClick">...</div>, and Angular can’t — don’t despair and fall back to addEventListener instead.


And that’s basically it — capture event listeners are useful for handling non-bubbling events (like image load / error) and for preventing certain events inside a container. There is an extra case I’ve used it in the past for — observing events when .stopPropagation is called by a third-party library, but it’s super hacky and not recommended. Trying to find a common theme, capture events work well wherever you feel like grabbing some DOM nodes you don’t own and slapping an event handler on them.

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Written in by your friend, Vladimir. Follow me on Twitter to get post updates. I have RSS, too. And you can buy me a coffee!
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