<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<textarea id="customInput" placeholder="Wpisz tutaj swój tekst!"></textarea>
<div id="charCount"></div>
<script>
var input = document.querySelector("#customInput"),
charCount = document.querySelector("#charCount");
var onKeyDown = function() {
charCount.textContent = input.value.length+1;
};
input.addEventListener("keydown", onKeyDown);
</script>
</body>
</html>