JSFiddle - React, Tailwind, and code Playground

HTML

<form name = "RegForm" class="login">
       <input type="text" name="username" id="username" placeholder="Enter your username">
    </form>

JavaScript

var username_textfield = document.forms.RegForm.username;
username_textfield.onkeydown = function(e) {
  var prohibited = "!@#$%^&*()+=;:`~\|'?/.><, \"";
  var prohibitedchars = prohibited.split("");
  var text = this.value.toString();
  var chars = text.split("");
  for (var i = 0; i < chars.length + 1; i++) {
    for (var a = 0; a < prohibitedchars.length + 1; a++) {
      if (chars[i] == prohibitedchars[a]) {
        chars[i] = null;
      }
    }
  }
  this.value = chars.join();
}