Auto extending HTML text area

autoExtendingTextArea.js

$('#ta').on('keydown',function(){
    var thisInput = event.target;
    var $thisInput = $(thisInput);
    var previousScrollHeight = $thisInput.data('previous-scroll-height');

    if(previousScrollHeight === null || previousScrollHeight === undefined){ //first time
      $thisInput.data('previous-scroll-height',thisInput.scrollHeight);
      previousScrollHeight = thisInput.scrollHeight;
    } 

    if(previousScrollHeight !== thisInput.scrollHeight){
      $thisInput.css('height', 'auto' );
      $thisInput.height( thisInput.scrollHeight );    
      $thisInput.data('previous-scroll-height', thisInput.scrollHeight);
      
    }

});

index.html

<div id='canvas'>
</div>
<textarea id='ta'></textarea>