怎样令js回调函数执行完再继续执行

Question

var username = $('#username');url="/index.php/login";$.post(url,{username:username.val(),type:'ajax'},function(result){if(result==1){$('#user_span').css('color','red');$('#user_span').html('用户名已存在');return false;}$(function(){$('#reg').live('submit',function(){checku();//这里有个函数var password=$('input[name="password"]');var confirm=$('input[id="confirm"]');//密码验证if(password.val()==""){password.next().css('color','red');password.next().html('密码不能为空');return false;}else{// 执行成功}//我点击submit时候checku()还没执行完,下面已经执行完