form表单提交前通过jq验证（ajax特别说明，return true失效）

<form id="form" action="{:U('Index/Messagesend')}" method="post" class="think-form" onsubmit="return

 

 <form id="form" action="{:U('Index/Messagesend')}" method="post" class="think-form" onsubmit="return toVaild()"></form>

function toVaild(){        var user = $("#user").val();        var content = $("#content").val();        var flat = false;        if(user != "" && content != "" ){            var url = "{:U('User/usercheck')}";            $.ajax({                async:false,                type:"post",                url :url,                data: {"user":user},                datatype:"json",                success:function(json){                    if(json != 1){                        alert('用户名不存在');                        flat = false;                    }else{                        flat = true;                    }                }            })        }else{            alert('请完成必填项');            return false;        }        return flat;    }

特别注意在ajax 的success:function你们返回 return true是不行的。