HTMLLinkElement where HTMLTextAreaElement expected

Question

I create an HTML page using PHP which contains textareas like these:

<div class='table-row'>
  <div class='table-cell-left'><!-- ------- instructions -->
    <label for='instructions' class='table-label'>short worst-case-scenario instructions.</label>
  </div>
  <div class='table-cell-right'>
    <textarea cols='50' name='instructions' placeholder='short worst-case-scenario instructions.' 
      maxlength='1000' onkeyup='checkLength(event,1000,"counter"," characters remaining");'
      class='timeText'>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
    </textarea>
  </div>
</div>

Not all of them have the "keyup" event, but all input elements share an event which I add at DomReady:

onDOMReady(function(){
  addHandlers();
});

function addHandlers(){
  var allElements = document.getElementsByTagName("*");
  for (i=0; i<allElements.length; i++) {
    switch (allElements[i].type && allElements[i].type.toLowerCase()){
      case "text":
      case "password":
      case "email":
        break;
      case "textarea":
        allElements[i].addEventListener("keyup", function(){
          checkHeight(allElements[i]);
        });
        checkHeight(allElements[i]);
        break;
    }
  }
}

function checkHeight(el){
  if (el.textLength > 0){
    el.style.height = "25px";
    el.style.height = (el.scrollHeight)+"px";}
  else{
    el.rows = 4;
  }
}

With this css:

textarea{
  display: block;
  float: right;
  resize: none;
  overflow: hidden;
}

all textarea's should be stretched to a minimum of 25px showing all the text in them. If there is no text an empty area of 4 lines is shown.

Now the problem: even though all handlers are correctly added, all textarea heights are correctly set on load, and a change in any textarea activates the checkHeight function, the object type is HTMLLinkElement, instead of HTMLTextAreaElement (which does not have the textLength attribute so the rows are set to 4 but the height is not changed). Also the element type is image/x-icon which makes absolutely no sense to me. Can anyone explain this to me? And maybe suggest a better way?

edit: OK, duplicate is clear, although I never found this in searching. 2 solutions:

for (let i=0; i<allElements.length; i++) { ...

or

case "textarea":
    allElements[i].addEventListener("keyup", function(){
      checkHeight(this);
    });

Thanks!