JSFiddle - React, Tailwind, and code Playground

HTML

<input type="number" id="field" keyup="keyUpFunction" step="0.01" placeholder="Field for both comma separators">

<span id="result" >
 <span >Current value: </span><span id="number"></span>
</span>

CSS

#result{
  padding: 10px;
  background-color: orange;
  font-size: 25px;
  width: 400px;
  margin-top: 10px;
  display: inline-block;
}

#field{
  padding: 5px;
  border-radius: 5px;
  border: 1px solid grey;
  width: 400px;
}

JavaScript

var currentString = '';
var badState = false;
var lastCharacter = null;

document.getElementById("field").addEventListener("keyup",($event) => {
  if ($event.target.value && !$event.target.validity.badInput) {
          currentString = $event.target.value;
  } else if ($event.target.validity.badInput) {
    if (badState && lastCharacter && lastCharacter.match(/[\.,]/) && !isNaN(+$event.key)) {
      $event.target.value = ((+currentString) + (+$event.key / 10));
      badState = false;
    } else {
      badState = true;
    }
  }
  document.getElementById("number").textContent = $event.target.value;
  lastCharacter = $event.key;
});

document.getElementById("field").addEventListener("blur",($event) => {
	if (badState) {
    $event.target.value = (+currentString);
    badState = false;
		document.getElementById("number").textContent = $event.target.value;
  }
});