Question

I’m struggling with the syntax for this user script I’m trying to write, which should basically follow this logic:

IF (LeftArrowKeyIsPressed) THEN doSomething;
ELSE IF (RightArrowKeyIsPressed) THEN doSomethingElse;
ELSE IF (any other key pressed) THEN doNothing;

Can someone help me out with the code syntax for structuring the check for the keypress? Assume an onkeydown event is being used.

Answer 1

Add an event listener, probably to the window, using addEventListener. Then switch off the Event.which value to do what you want.

Here’s the code. See it in action at jsFiddle.:

/*--- Get or set the codes for the arrow keys.
    Firefox gives us nice constants, Chrome does not.
*/
KeyEvent        = (typeof KeyEvent === "object")  ?  KeyEvent  :  [];
const LEFT_KEY  = KeyEvent.DOM_VK_LEFT   ||  37;
const RIGHT_KEY = KeyEvent.DOM_VK_RIGHT  ||  39;

window.addEventListener ("keydown", keyboardHandler, false);

function keyboardHandler (zEvent) {
    var bBlockDefaultAction = false;

    //--- Assume we want only the plain keys, not the modified versions.
    if (zEvent.altKey  ||  zEvent.ctrlKey  ||  zEvent.shiftKey) {
        //-- Do nothing (most user-friendly option, in most cases).
    }
    else {
        if (zEvent.which == LEFT_KEY) {
            //DO LEFT KEY ACTION HERE.
            bBlockDefaultAction = true;
        }
        else if (zEvent.which == RIGHT_KEY) {
            //DO RIGHT KEY ACTION HERE.
            bBlockDefaultAction = true;
        }
    }

    if (bBlockDefaultAction) {
        zEvent.preventDefault ();
        zEvent.stopPropagation ();
    }
}

---

This code works on userscripts-applicable browsers (not IE). The jQuery version is the same except change the addEventListener line to:

$(window).keydown (keyboardHandler);