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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");
}
});

Input wont return true when using Javascript

Attempting to log the input that the following gives me. But I am unable to even get anything other then false when attempting to type in the box. As in i'm able to type a single character, have it return true and then almost immediately it choses to no longer respond and always returns false no matter what I type inside of the box.
function search() {
var value = document.getElementById("search").value;
console.log(value == ''); // true
}
<input type="text" onkeydown="search()" id="search"></input>
Testing - Chrome 40.0.2214.94 (64-bit)

Problem is your trying to compare strings. When you do the comparison with a == it is looking at values. You need to use === then you will be able to check if the input is equal to whatever you are searching for.

Here is the updated JSFiddle
You have to use keyup event because you want result after the key is pressed.

Keyboard events is one of wilder parts of frontend development. There are inconsistencies and cross-browser problems.
But still there are recipes which help to cope with ordinary situations easily.
There are following keyboard events:
keydown
A key is pressed down.
keypress
A character key is pressed.
keyup
A key is released.
There is a fundamental difference between keypress and keydown.
Keydown triggers on any key press and gives scan-code.
Keypress triggers after keydown and gives char-code, but it is guaranteed for character keys only.
You need to use keyup, because it triggers after keypress.
<input type="text" onkeyup="search()" id="search"></input>
see KeyEvents

hash (#) vs end charCode from keypress event in Firefox

In Firefox (8) both the end key and # key have the same charCode (35). Is there a way to tell them apart?
I made a quick demo on jsfiddle. Just type in the input box and it will show you the charCode/key that you pressed.
http://jsfiddle.net/PfAeW/
I'm building a rich text editor where # triggers an autocomplete function. I need to cater to international users (UK keyboards) so looking to see if shift is being pressed would not work.

There are two kinds of codes for key events in the DOM: charCode, which represents the character for printable keys and keyCode which represents the physical key pressed on the keyboard.
It happens that the charCode of '#' is 35 and the keyCode of the end key is also 35, but they come with totally different number-to-meaning mappings.
Mootools exposes a .code on events which conflates the two codes, hence the confusion.

Just so there's an answer...
It looks like Mootools is mis-representing the keypress (which on your test also show mis-mappings of the following as well:
Key Pressed How Moo Sees It
----------- ---------------
end # 35
. (period) delete 46
( (left paren.) down 40
' (apos.) right 39
But, you may be able to use the Keyboard (coupled with the Keys list) and bind to the correct sequence. I'm not fluent in moo-tools, but given the document examples I don't see why the following wouldn't work:
var kb = new Keyboard({
defaultEventType: 'keypress'
});
kb.addEvents({
'end': fnEnd,
});
Disclaimer: More-or-less showing alternative (or so is my understanding based on docs) than solution. I do not know if the above code works, but it should. I am, however, not sure how you'd bind the above to a specific control, so that may be one limitation that Keyboard has over a direct binding.

You can checking for shift (event.code == 16) key in keydown event and preserve value of them and when keypress will be triggered you are sure if code from keydown event was 16 and now is 35 # really is pressed. Uless there is another way to check the shift is pressed:)

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.)