How to apply field validation without using JQuery

Question

I am trying to only allow 9 digits in a single field without using JQuery. I am unable to modify html input. Thought this would be simple without JQuery but it doesn't seem that way. Any assistance would be appreciated.

Fiddle: https://jsfiddle.net/wj_fiddle_playground/b46tpaLj/4/

HTML Below:

<input class="formInput" type="text" style="width: 200px;" name="myNumber" id="myNumber" value="">

Javascript Below

function setInputFilter(textbox, inputFilter) {
  ["input", "keydown", "keyup", "mousedown", "mouseup", "select", "contextmenu", 
"drop"].forEach(function(event) {
 textbox.addEventListener(event, function() {
  if (inputFilter(this.value)) {
    this.oldValue = this.value;
    this.oldSelectionStart = this.selectionStart;
    this.oldSelectionEnd = this.selectionEnd;
  } else if (this.hasOwnProperty("oldValue")) {
    this.value = this.oldValue;
    this.setSelectionRange(this.oldSelectionStart, this.oldSelectionEnd);
  } else {
    this.value = "";
  }
  });
  });
}


setInputFilter(document.getElementById("myNumber"), function(value) {
  return /^\d{9}/.test(value); });