In Javascript, how do I tell if a user is pressing two keys at the same time?

Question

In Javascript, how do I tell if a user is pressing two keys at the same time?

For example, I have drawn a circle in the middle of the screen. I want to move it up while

Answer 1

Javascript has onkeydown, and onkeyup events. You can simple fiddle a bit onkeydown for left arrow, and fiddle another bit for the up arrow. On keyup, fiddle the respective bit back.

var leftPressed,
    upPressed,
    rightPressed,
    downPressed;

var milli = 100;

window.addEventListener('onkeydown', function(e) {
    switch(e.keycode) {
        case 37: //left
            leftPressed = true;
        case 38: //up
            upPressed = true;
        case 39: //right
           rightPressed = true;
        case 40: //down
            downPressed = true;
        default:
            break;
    }
});

window.addEventListener('onkeyup', function(e) {
    switch(e.keycode) {
        case 37: //left
            leftPressed = false;
        case 38: //up
            upPressed = false;
        case 39: //right
           rightPressed = false;
        case 40: //down
            downPressed = false;
        default:
            break;
    }

});

function moveCircle() {
    if(leftPressed && !rightPressed) {
        // move left
    }
    if(rightPressed && !leftPressed) {
        // move right
    }
    if(upPressed && !downPressed) {
        // move up
    }
    if(downPressed && !upPressed) {
        // move down
    }
}

setInterval(moveCircle, milli);