JSFiddle - React, Tailwind, and code Playground

HTML

Type in here and hold down a key:
<br>
<input type="text" id="my_textbox">

JavaScript

var textBox = document.getElementById("my_textbox");

var lastKeyDown, keyIsPressed = {}, keyCodeForCharCode = {},
    charCodeForKeyCode = {};

textBox.onkeydown = function(evt) {
    evt = evt || window.event;
    lastKeyDown = evt.keyCode;
};

textBox.onkeyup = function(evt) {
    evt = evt || window.event;
    var charCode = charCodeForKeyCode[evt.keyCode];
    if (charCode) {
        keyIsPressed[charCode] = false;
    }
};

textBox.onkeypress = function(evt) {
    evt = evt || window.event;
    var keyCode, charCode = evt.which || evt.keyCode;

    if (keyIsPressed[charCode]) {
        // This keypress is an autorepeat
        return false;
    } else {
        keyIsPressed[charCode] = true;

        // Get the key code for the corresponding keydown event
        keyCode = keyCodeForCharCode[charCode];
        if (!keyCode) {
            // Create two-way mapping for keyCode and charCode
            keyCodeForCharCode[charCode] = lastKeyDown;
            charCodeForKeyCode[lastKeyDown] = charCode;
        }
    }
};