为什么Javascript返回[object Object]？

<script type="text/javascript">
  $("#sign_up").on('click', function() {
    $.post('./includes/ajax.php', { action: 'register' } , function(result) {
      var result = JSON.parse(result);
        if(result ) { $("#register_result") = result; document.write(result); }
    });
  });
$("#register_form").submit(function() {
  return false;
});
</script>
<script type="text/javascript">
  $("#sign_up")