需要实现的功能:

在注册前对用户名及电子邮箱进行验重。
由于需要明确的返回注册失败原因，因此并未使用or, 而是使用了两次查询。

代码如下:

dbutil.query('select id from user where username="拭目以待"', function (err,data){
    if(data.length !== 0){
        goBack('用户名被占用');
    }
});
dbutil.query('select id from user where email="182209508@qq.com"', function (err,data){
    if(data.length !== 0){
        goBack('该邮箱已注册');
    }
});
function goBack(msg){
    var errorJSON = {
        status: 'error',
        msg: msg
    };
    res.write(JSON.stringify(errorJSON));
    res.end();
}

执行后的错误信息:

Error: write after end
    at ServerResponse.OutgoingMessage.write (_http_outgoing.js:426:15)
    at goBack (/Users/baukh/work/baukhZone/exports/userManager.js:109:21)
    at /Users/baukh/work/baukhZone/exports/userManager.js:96:21
    at Query._callback (/Users/baukh/work/baukhZone/dbutil.js:36:33)
    at Query.Sequence.end (/Users/baukh/work/baukhZone/node_modules/mysql/lib/protocol/sequences/Sequence.js:85:24)
    at Query._handleFinalResultPacket (/Users/baukh/work/baukhZone/node_modules/mysql/lib/protocol/sequences/Query.js:144:8)
    at Query.EofPacket (/Users/baukh/work/baukhZone/node_modules/mysql/lib/protocol/sequences/Query.js:128:8)
    at Protocol._parsePacket (/Users/baukh/work/baukhZone/node_modules/mysql/lib/protocol/Protocol.js:280:23)
    at Parser.write (/Users/baukh/work/baukhZone/node_modules/mysql/lib/protocol/Parser.js:74:12)
    at Protocol.write (/Users/baukh/work/baukhZone/node_modules/mysql/lib/protocol/Protocol.js:39:16)

经过排查发现: 同时执行多个数据库操作，在只有一个返回结果时，将请求跳出就会导至这种[Error: write after end]错误。

修改后的代码：

dbutil.query('select id from user where username="拭目以待"', function (err,data){
    if(data.length !== 0){
        goBack('用户名被占用');
    }
    else{
        dbutil.query('select id from user where email="182209508@qq.com"', function (err,data){
            if(data.length !== 0){
                goBack('该邮箱已注册');
            }
        });
    }
});
function goBack(msg){
    var errorJSON = {
        status: 'error',
        msg: msg
    };
    res.write(JSON.stringify(errorJSON));
    res.end();
}