var isCtrlPressed = false;

​

function onKeyDown(event) {

    if (event.keyCode == 17) { // Control got pressed

        isCtrlPressed = true;

    }

    if (event.keyCode == 83) { // "s" got pressed

        // if control is pressed too execute some code

        if (isCtrlPressed) {

            // Your code here

            alert("Ctrl+s pressed");

        }

    }

}

​

function onKeyUp(event) {

    if (event.keyCode == 17) { // Control got pressed

        isCtrlPressed = false;

    }

}