<textarea maxlength="10" resize="none" name="tweet" id="textbox" class="form-control" rows="10" cols="60"></textarea>
<span id="char_count"></span>
<script>
var count_chars = document.getElementById("textbox").maxLength;
let textArea = document.getElementById("textbox");
let characterCounter = document.getElementById("char_count");
characterCounter.textContent ="Осталось: " + count_chars + " из " + count_chars;
const maxNumOfChars = count_chars;
const countCharacters = () => {
let numOfEnteredChars = textArea.value.length;
let counter = maxNumOfChars - numOfEnteredChars;
characterCounter.textContent ="Осталось: " + counter + " из" + count_chars;
};
textArea.addEventListener("input", countCharacters);
</script>