Code & detects ctrl+z

document.onkeyup = function(e) {
if(e.ctrlKey && e.keyCode == 90) {
// ctrl+z pressed
}
}

jQuery also has an excellent implementation that's incredibly easy to use. Here's how you could implement this functionality across browsers:

$(document).keypress(function(e){
var checkWebkitandIE=(e.which==26 ? 1 : 0);
var checkMoz=(e.which==122 && e.ctrlKey ? 1 : 0);


if (checkWebkitandIE || checkMoz) $("body").append("<p>ctrl+z detected!</p>");
});

Tested in IE7,Firefox 3.6.3 & Chrome 4.1.249.1064

Another way of doing this is to use the keydown event and track the event.keyCode. However, since jQuery normalizes keyCode and charCode using event.which, their spec recommends using event.which in a variety of situations:

$(document).keydown(function(e){
if (e.keyCode==90 && e.ctrlKey)
$("body").append("<p>ctrl+z detected!</p>");
});

For non-printable keys such as arrow keys and shortcut keys such as Ctrl-z, Ctrl-x, Ctrl-c that may trigger some action in the browser (for instance, inside editable documents or elements), you may not get a keypress event in all browsers. For this reason you have to use keydown instead, if you're interested in suppressing the browser's default action. If not, keyup will do just as well.

Attaching a keydown event to document works in all the major browsers:

document.onkeydown = function(evt) {
evt = evt || window.event;
if (evt.ctrlKey && evt.keyCode == 90) {
alert("Ctrl-Z");
}
};

For a complete reference, I strongly recommend Jan Wolter's article on JavaScript key handling.

Detect key press, including key combinations:

window.addEventListener('keydown', function (e) {
if (e.ctrlKey && e.keyCode == 90) {
// Ctrl + z pressed
}
});

Benefit here is that you are not overwriting any global properties, but instead merely introducing a side effect. Not good, but definitely a whole lot less nefarious than other suggestions on here.

For modern JS, use event.key!

document.addEventListener("keypress", function onPress(event) {
if (event.key === "z" && event.ctrlKey) {
// Do something awesome
}
});

NOTE: The old properties (.keyCode and .which) are Deprecated.

Mozilla Docs

Supported Browsers

Attach a listener to the keydown event instead of keypress, since the latter is now deprecated.

window.addEventListener('keydown', keyDownHandler);

The keydown event triggers continuously while the key is pressed. If you wanna have it fire only once, inside the handler use the event.repeat property as so:

keyDownHandler(event) {
if (!event.repeat) {
<code here will only be executed once while the key is pressed>
}
}

Remember to remove the listener when not needed anymore.

window.removeEventListener('keydown', keyDownHandler);