Force keydown event to fire only once per keyCode

Question

Situation: I have a `keydown` handler with a `switch` for what key is pressed, fairly standard stuff, but when any key is held down the `keydown` event fires repeatedly (rather than just once when the key is actually pressed). Why it's a problem: I want to keep the `keydown` listener active, i.e. be able to detect more than one key being pressed at once, but only have the event fire once per `keydown`. I will want to do something on the `keyup` for that `keyCode` based on the time between down and up, but that timing is getting screwed up because of the multiple firings. What I've tried: I am currently keeping a list of `keyCodes` that are down, and checking against those in my `keydown` handler to keep the default behaviour from happening if the `keyCode` is in my list. However, the event is still firing very often and I'm concerned about the efficiency/elegance of this solution. The actual question: Is there a good way to limit firing of the `keydown` event to just when the key is p…

Accepted Answer

In the keydown handler, check whether the key is the same as the last key handled. If it was, exit early. On keyup, forget the last key handled.

For example:

```
var lastEvent;
var heldKeys = {};

window.onkeydown = function(event) {
    if (lastEvent && lastEvent.keyCode == event.keyCode) {
        return;
    }
    lastEvent = event;
    heldKeys[event.keyCode] = true;
};

window.onkeyup = function(event) {
    lastEvent = null;
    delete heldKeys[event.keyCode];
};​
```

Demo here (click on the "Result" panel and hit keys).

Force keydown event to fire only once per keyCode

Solution

Problem