You receive two keydown events: The first is for the control key, and the second is for the letter with the modifier flag turned on. Here’s how you listen for a key with a modifier pressed:
document.addEventListener("keydown", function(e) {
if (e.keyCode === 83 && (navigator.platform.match("Mac") ? e.metaKey : e.ctrlKey)) {
e.preventDefault();
// Process event...
}
}, false);
keyCode === 83
corresponds to S key press.
Taking a page from Google Docs, it uses Cmd-S on Mac and Ctrl-S on other platforms.