Limit Character Count in WP Content Editor

admin.js

var postTypeProject = document.getElementsByClassName('post-type-post'),
    projectEditor = document.getElementById('content'),
    charCount = 460;

if(postTypeProject[0] && projectEditor){
  var publishBtn = document.getElementById('publish'),
      theValue = projectEditor.value,
      contentWrap = document.getElementById('wp-content-wrap'),
      message = document.createElement('div'),
      errorBox = contentWrap.getElementsByClassName('message-length'),
      messageText;
  function onPageLoad(){
    if(theValue.length > charCount){
      publishBtn.setAttribute('disabled', 'disabled');

      if(errorBox[0]){
        projectEditor.value = theValue.substring(0, theValue.length - 1);
      } else {
        messageText = document.createTextNode('Content must have less than 460 characters, currently ' + theValue.length);
        message.className = 'error notice message-length';
        message.appendChild(messageText);
        contentWrap.appendChild(message);
      }

    } else if(publishBtn.getAttribute('disabled')) {

      publishBtn.removeAttribute('disabled')

    } else {
      false;
    }
  }
  onPageLoad();

  function onChange(){
    theValue = projectEditor.value;
    if(theValue.length > charCount){
      publishBtn.setAttribute('disabled', 'disabled');

      if(errorBox[0]){
        projectEditor.value = theValue.substring(0, theValue.length - 1);
      } else {
        messageText = document.createTextNode('Content must have less than 460 characters, currently ' + theValue.length);
        message.className = 'error notice message-length';
        message.appendChild(messageText);
        contentWrap.appendChild(message);
      }

    } else if(publishBtn.getAttribute('disabled')) {

      publishBtn.removeAttribute('disabled')

    } else {
      false;
    }

  }

  function onInput(){
    theValue = projectEditor.value;
    if(theValue.length > 460){

      publishBtn.setAttribute('disabled', 'disabled');

      if(errorBox[0]){
        projectEditor.value = theValue.substring(0, theValue.length - 1);
      } else {
        messageText = document.createTextNode('Content must have less than 460 characters, currently ' + theValue.length);
        message.className = 'error message-length';
        message.appendChild(messageText);
        contentWrap.appendChild(message);
      }
    } else if(publishBtn.getAttribute('disabled')){
      publishBtn.removeAttribute('disabled');
    } else {
      false;
    }
  }

  projectEditor.addEventListener("input", onInput, false);
  projectEditor.addEventListener("change", onInput, false);
}

function.php

<?php
  function admin_enqueue( ) {

      wp_enqueue_script( 'post_editor_limit', get_template_directory_uri() . '/assets/admin/js/app.js', false, null, true );
  }

  add_action('admin_enqueue_scripts', 'admin_enqueue');