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How to detect actual characters only with JS key-events (for text-editor)

I am working on a WYSIWYG-like editor for equations (in combination with plain text).
Since implementing the functionalities (for equations) that I'd like to have won't work using the existing frameworks (contentEditable or document.designMode), I am now building an editor from scratch. (so far it has worked out good, I've successfully implemented most functionalities of a normal editor)
I've been using the keydown event to detect user input while in "edit-mode" (that is, when the user has clicked on the editor-area, also displaying the cursor), but the problem with that is, that when clicking "alt" or "strg" or other keys that aren't actual characters, they'll also get displayed in my editor.
Now, what I've tried is to ignore those keys by using if-statements, but there are 2 issues I see with that:
1. It may influence performance, when too many keys have to be ignored
2. I can never be sure, if there doesn't exist some exotic key, perhaps on a Mac or so, which I didn't ignore
I have also tried to use the keypress event instead, which worked mostly fine, but still displayed "Enter" and "Delete", so who knows what others keys it may display too. Also, it is labelled as "deprecated".
Are there any better ways of doing that, or will I just have to make a big list of keys to ignore?

A simple (but limited) approach would be to check if the keydown event's keyCode is between 65 (code for key 'a') and 90 (code for 'z').
If you need more than just letters though, another solution would be to check the event's key and its length. For actual characters, the key simply holds that character (so length 1). For Ctrl, Shift, etc., key will hold the full name of the key, e.g. "Control", "Shift", etc.
So, if a key is an actual character, the key property will have a length of 1.
document.onkeydown = function(e) {
var keycode = e.keyCode;
if(e.key.length == 1) {
document.querySelector("#editor").innerHTML += e.key;
}
}
<p id="editor"></p>

You can check this one too
document.addEventListener("keydown", (event) => {
console.log(event);
if (event.code === "Escape") {
console.log("abc");
}
});

javascript - calculate "true" keyCode

When I press a key on my keyboard, I can get the keyCode of that key using e.keyCode. But the keyCode I get does not consider the pressed alt/ctrl/shift keys, wich modify the key code.
Fortunately, I get the properties shiftKey/altKey/ctrlKey with the event so I am able to calculate the "true" key code.
But I don't know how to calculate this correctly. Where can I read about that?
Is it as easy as substracting 32 if shift ist pressed and so on, or are there much exceptions?

You're mistaken, key codes don't change because they refer to a specific key on the keyboard. Regardless of modifier keys, those codes stay the same (and even the modifier keys themselves have a key code).
Capture the keypress event, which will allow you to access character codes.
el.onkeypress = function (evt) {
alert( (evt || window.event).charCode );
}

I don't understand why you would like to calculate the "true" keyCodes. If you are making shortcuts for your site/app/whatever, you could simply check if the button was pressed was (for example) S and that Ctrl was pressed at the same time.
True keyCodes have no meaning by itself (unless you need it for something specific).
Btw, you should consider using jQuery for your project. It normalizes the keyCodes so you don't have any weird behavior in any browser (Windows and OS). I guess that is more important.

crossbrowser keypress for special keys (arrows,...) in javascript

I am building a browser interface to a terminal. I need to catch both character (alphanumeric, dot, slash,...) and non-character key presses (arrows, F1-F12,...). Also, if the user holds some key down, it would be nice to get repeated keypresses (the function should be called repeatedly until key is released). The same goes for space key, characters,...
I want this to be as cross-browser as possible (jQuery keypress fails on that account). I have also tried using fork of jquery.hotkeys.js, but if I understand correctly, I can't catch both special and character keys in a single function (one should use keydown for former and keydown for the latter).
Is there a JS library that would allow me to catch both character and special keys?
I hope I'm not missing something obvious. :)
UPDATE To clarify: I am looking for the library that would hide the browser implementation details from me.

The onkeydown it exactly what you need. It captures all keys, even if you are holding a button it is fired repeatedly.
<input type='text' onkeydown='return myFunc(this,event)'>
<script>
function myFunc(Sender,e){
var key = e.which ? e.which : e.keyCode;
if(key == someKey){ // cancel event and do something
ev.returnValue = false;
if(e.preventDefault) e.preventDefault();
return false;
}
}
</script>
UPDATE try and test this with jQuery
$(document).ready(function(){
$('#in').keydown(fn);
});
var cnt = 0;
function fn(e){
var key = e.keyCode ? e.keyCode : e.which;
cnt++;
if(cnt == 10) {
alert('event was fired 10 times. Last time with key: '+key);
cnt = 0;
}
}

The DOM 3 Events spec includes key events, but it's still a working draft so likely not that widely supported yet but should be pretty helpful.
For turning key codes into characters, you might find Quriskmode helpful. Knowing which key was pressed and which modifiers should get you where you want to be. Note that you may have issues mapping all keyboards to the same character sets because some have keys that others don't (e.g. Microsoft "windows" key and Apple command key). A bit of trial and error might be required.
Oh, and you might find the article JavaScript Madness: Keyboard Events interesting.

I ended up using keycode.js, but am building a whole event-managing system around keydown, keypress and keyup events, because just one of the events (keydown) is not enough to determine which character was entered and which key was pressed if there is no corresponding character. Browser incompatibilities are an added bonus to this challenge. :)
Thank you both for your answers, it has helped me understand the problem fully.

Detecting command + keystroke in Safari

I'm trying to intercept the command + keystroke in Safari. I've added an event handler as follows:
document.onkeypress = handleKeyPress;
function handleKeyPress(event) {
if ("+" === String.fromCharCode(event.charCode) && event.metaKey) {
// my code here
return false;
}
return true;
}
When I hit command shift = (shift = is + on my US keyboard), the if statement does not return true.
If I remove the event.metaKey portion of the if statement and hit shift =, the if statement does return true.
Also, if I change the matching string from "+" to "=" and hit command = (with or without the shift key), the if statement does return true.
Is there a way to actually detect the command + keypress (without assuming that the + key is shift = and checking for the event.shiftKey, since this will not be true for some non-US keyboards)?

First of all, I can't necessarily recommend using ⌘+ as a shortcut, since it already means something in Safari (namely, zoom the page). Someone might want that to still work normally. Depending on what you're building, it might make sense, but don't override the default shortcuts unless you're sure.
Anyway: Key events are tricky, often because the keyCode/charCode/which properties don't always match up with the right letter, so String.fromCharCode won't always get you a proper string. keyIdentifier is sometimes a better thing to look at, but not always (for instance, its codes for letter keys are always uppercase letters).
What I've done in the past (and I'm not sure that this is the best way to do it, but it works ok), is to instead listen for keydown and keyup events, and "stack" modifier keys. I.e. whenever a key is depressed, check whether it's a modifier key (i.e. if it's cmd, ctrl, alt, or shift). If it is, add it to the "stack" of modifiers. Do the opposite on keyup; if the released key was a modifier, remove it from the stack.
Now, back in the keydown-handler, if the key wasn't a modifier key, you can send the keydown event (which, unlike a keypress event, will have a pretty reliable keyIdentifier property to check), and the stack of modifiers along to some other callback, that'll take it from there.
In your case, such a callback would check that the cmd-key is in the stack, and that the keyIdentifier for the keydown event is "U+002B" (which is the unicode code for +).
I've put together a jsfiddle example of what I'm talking about. If you click in the "Result" pane (to make sure it's got focus), and press ⌘+, it should show the key combo you used, and write "Success!" below. Otherwise, it'll just show the key combo. On my keyboard, the plus sign is directly accessible, so the key combo I see is just "⌘+". But if you need shift to type a plus sign, you should see "⇧⌘+".
It's a generalized piece of code that's good for handling keyboard shortcuts in Safari/Mac, so you can build on it, if you want. You'll want to add a few event listeners to reset the modifier stack on blur events and such. Ideally, the modifier stack would reset automatically, as you release the keys, but since something might cause the browser, or the observed element/window/document, to lose focus, the keyup events won't be handled, and the released keys won't be removed from the stack. So check for blur events.

Register the event object in a window object level one, event = window.EE for example, then browse it with Firebug or Safari’s Web Developer Tools and see the values that are triggered when you want. So you will know what to compare against.

Sadly, I don't think there is an answer. The problem is, for CMD+SHIFT+= the charCode being pressed is 61 (=), not 43 (+). This appears to be a system-wide design, in that Mac OS X itself interprets CMD+SHIFT+= as = plus two modifiers, not + and COMMAND.
I put together a simple jsFiddle to show this: http://jsfiddle.net/ScuDj/1/
Basically, you are going to have to deal with keyboard layouts if you want to detect COMMAND +.
Alternatively, you could only support the numeric keypad + - that works consistently! (just kidding ;-)
(Also: I tried to attach to the textInput event, but couldn't get it to register globally. I think it only works on domNodes. I've not really used that event much.)

get character typed, cross-browser

Simple question -- Does anyone know of a reliable cross-browser function to get the character typed from a keydown event? I can write one from the quirksmode grid but would rather not re-invent the wheel for something so simple yet so non-standard.
Let me clarify:
There is no way to do this simply using event.keyCode or event.which. The overall logic is something like:
get keycode
detect shift, ctrl
if ctrl, ignore
create map of keycodes with and without shift key
example map {186 : ';', 187 : '=', 188 : ',', ....}
need a seperate map for shift key modifier
map needs to change depending on browser (especially Safari Mac which has keycodes like 60,000)
if keycode is in map, return mapped key
if keycode is not in map, normalize numbers, apply shift modifier key if necessary, return String.fromCharCode(key)
It's not a simple solution, which is why I'm looking for a pre-made one :)

Are you willing to use jQuery?
$("input").bind("keydown",function(e){ var value = this.value + String.fromCharCode(e.keyCode); }
I believe that switching to the keypress event would solve the issue mentioned in your comment. Would the code below meet your requirments?
$("input").bind("keypress",function(e){ var value = this.value + String.fromCharCode(e.keyCode); }

I've been wondering the same thing for years. Recently I got fed up with having to look up keycodes for events that I wrote a module called keysight that translates keypress, keydown, and keyup events into characters and keys respectively.
Example:
element.addEventListener("keydown", function(event) {
var character = keysight(event).char
})
It doesn't ignore anything on ctrl, so you'd have to do that manually if you want. But it does all the rest of the logic you gave.