How to handle keyup events in game loop?

Question

I'm writing a simple game in JavaScript, which already handles basic keyboard input like so:

var input = {};
while (!done) {
    handleInput(input);
    update();
    render();
}

document.onkeydown = function(e) {
    input[e.keyCode] = true;
}
document.onkeyup = function(e) {
    input[e.keyCode] = false;
}

Now I need the game to handle key combos (like CTRL+X for example). I would like it to accept such combos on keyup only.

Two possible solutions that come into my mind are:

• exposing an array containing a list of keyup events (object with "main" key plus modifiers). The handleInput function would be responsible for draining the queue every time it polls it
• keeping track of possible key combos inside handleInput (watching for held down modifier keys) and trigger the combo behavior when the "main" key goes up (I actually don't like this that much)

Would you suggest me an elegant way to extend the current functionality?

Answer 1

It sounds like you need a enum for state rather than boolean. My suggestion is something like [NONE (or something else), PRESSED, HELD, RELEASED]. And in the game loop create a function to switch any keys that are pressed to held and any keys that are released to none.
Also you might run into issues (especially with this implementation) where your input changes mid handleInput() and I suggest adding a queue to "lock" input to the beginning or end of the loop.