I'm bit of a novice to JavaScript so I've been wondering on positive and negative sides of using polling compared to using events in JavaScript? When does one use one or the other?

By polling I consider any effect that is triggered on something that repeats itself usually using setTimeout. But also for example:

function render() {
   Bloop.renderComponent(box, document.body);

Is also polling since requestAnimationFrame(render) which is called before each frame is called.

Basically anything that fits this definition


This is a bit of a "which is better, fish or the colour orange" type of question, but in terms of the general behaviour of JavaScript, and particularly the more functional side of it's nature it is probably more idiomatic to use events to react to changes of state than it is to have a timed polling system.

It is a lot more flexible and less confusing to be adding and removing event listeners as your application starts to grow. Polling is easier to conceptualise when you are first imagining everything clocking in to a central location, but as your application gets more complex that is liable to get increasingly difficult to do, whereas events can come from anywhere and go to anywhere so they are much less tightly coupled.

Also when an event is triggered and there is nobody to hear it, nothing happens. If you have a whole lot of timeout based polling loops, you may find that things are happening after the main system crashed as timeouts trigger, which makes debugging the problem or following up any logs more confusing.

Practically if you are animating things, you will probably want to use some degree of polling for your animation loop and events to communicate when to trigger other animations or when interactions occur, so you're likely to want to use both.

  • Yeah, I'm aware that the question is strange. Just wanted to have a better grasp of pros and cons. – Daniel Fath May 15 '14 at 20:34

In general, my preference is to base the design on polling at fairly low frequency, but then to use events to trigger early scans.

The good thing about events is responsiveness.

There are a few bad things about them:

  • They can happen more often than you want, as for example there may be 10 events in a row, early ones being overridden by later ones, and the program gets jerked around trying to track every little change. This is the "short leash" problem. If there are two bodies of information, that need to be kept in agreement, I typically use a routine to scan for disagreements and patch them up, rather than count on events or notifications.

  • Events can be dropped or duplicated, so if it is important to keep track of changes, they can lead to errors. This is the "why wasn't I told?" or "did you already tell me that?" problem.

  • It is easy to create event handlers, so easy that it is easy to create more than you want. This can lead to things like slow window painting because it's being done many times when it only needed to be done once. Then when you remove an event handler, you may think you've turned off the event handling, but you haven't, because you haven't removed all of them. This is the "spam" problem.

So as I said, I prefer to minimize event handling (and its cousins - notifications and nested properties), but to definitely employ them, sparingly, where their benefits will do the most good.

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