How to detect if the pressed key will produce a character inside an <input> text-box?

This I think will do the job, or if not is very close and will need only minor tweaking. The thing you have to remember is that you can't reliably tell anything at all about any character that may be typed in a keydown or keyup event: that all has to be done in a keypress handler. The definitive resource for key events is http://unixpapa.com/js/key.html

You also need to consider pastes, which this code won't handle. You will need to have separate paste event handler (although this event isn't supported in Firefox < 3.0, Opera, and very old WebKit browsers). You'll need a timer in your paste handler since it's impossible in JavaScript to access the content that's about to be pasted.

function isCharacterKeyPress(evt) {
    if (typeof evt.which == "undefined") {
        // This is IE, which only fires keypress events for printable keys
        return true;
    } else if (typeof evt.which == "number" && evt.which > 0) {
        // In other browsers except old versions of WebKit, evt.which is
        // only greater than zero if the keypress is a printable key.
        // We need to filter out backspace and ctrl/alt/meta key combinations
        return !evt.ctrlKey && !evt.metaKey && !evt.altKey && evt.which != 8;
    }
    return false;
}

<input type="text" onkeypress="alert(isCharacterKeyPress(event))">

Here's a much simpler solution which worked well for me:

document.addEventListener('keyup', event => {
  if (String.fromCharCode(event.keyCode).match(/(\w|\s)/g)) {
    //pressed key is a char
  } else {
    //pressed key is a non-char
    //e.g. 'esc', 'backspace', 'up arrow'
  }
});

This doesn't require probing a DOM element (which would add latency and ugliness).

Updated example use:

The possible solution I can find is checking the length of key in event.

Eg:-

<input type="text" id="testId" onkeyup="keyChecking(event)" />

<script type="text/javascript">
function keyChecking(event) {

    if (event.key.length == 1) {
        alert("key produced character " + event.key);
    } else {
        alert("Key DOES NOT produce character");

        const alphabets = "AZaz09";
        const key = event.key;
        var notEvenASymbol = false;

        for (let i = 0; i < key.length; i++) {
            var charCode = key.charCodeAt(i);
            if ((charCode >= alphabets.charCodeAt(0) && charCode <= alphabets.charCodeAt(1)) ||
                (charCode >= alphabets.charCodeAt(2) && charCode <= alphabets.charCodeAt(3)) ||
                (charCode >= alphabets.charCodeAt(4) && charCode <= alphabets.charCodeAt(5))
            ) {
                notEvenASymbol = true;
                console.log(charCode);
                break;
            }
        }

        if (notEvenASymbol) {
            alert("Key DOES NOT produce even a symbol");
        }
        console.log(event.key);

    }
}    
</script>

So, if you press any characters/symbols, the event.key will contain that character and its length will be 1. If you press character V then the event.key will have value V but if you press enter key then it will contain value Enter, if you press shift then Shift and so on. Therefore, if a key doesn't produce a character then its length will be greater than 1.

Updated

Some special keys in the keyboard produce symbol and its length may be greater than 1 so I modified the code so that it can alert even if it's not a symbol. Eg:- its length is 2. Some mobile keyboards have shortcut keys for such symbols.

A non character/symbol key in the keyboard will always be a combination of alphabets, number characters or of both, Eg:- F2, Shift.

Thanks @Vicky Chijwani for bringing attention to this scenario.