<form id="frm_user_form" onsubmit="return false">
  Username: <input type="text" name="txt_username" /><br />
  Password: &nbsp;<input type="password" name="txt_password" /><br />
  <button>Show Json</button>
</form>

$(function(){
   $("button").click(function(){
        var result = {};
        $.each($("#frm_user_form").serializeArray(),function(){
             result[this.name] = this.value;
        });
        alert(JSON.stringify(result));
   });
});