[Javascript] How to enlarge and small the font

html code:

<a href="####" id="zoom_minify">Zoom In</a>
<a href="####" id="zoom_magnify">Zoom Out</a>

js code:

<script type="text/javascript">
$('#zoom_minify').click(function(){
        textZoom(-1);
      });
  $('#zoom_magnify').click(function(){
    textZoom(1);
  });
  function textZoom(i){
    var newSize = Number($('body').css('font-size').substring(0, $('body').css('font-size').length-2)) + i;
    $('body').css('font-size', newSize);
  }
</script>