暂时没找到问题的所在。

	app.post('/chart', function(req, res){
				var SQL = 'xxxxxxx';
				connection.query(SQL, function(err, rows){
					res.send(rows);
				})
		})

如果这样写的话会报错： Can’t set headers after they are sent；

如果是写成 :

app.post('/chart', function(req, res){
	var SQL = 'xxxxxxxxxx';
	connection.query(SQL, function(err), rows){
			xxxxxxx
	};
	res.send({status:  0 });
})

这样是不会报错的。- -

前面的代码都没返回数据给前台的。

完整的代码：

var app = express();
var http = require('http').Server(app);
var io = require('socket.io')(http);
//var server = http.createServer(app);
http.listen(10086);

app.use(bodyParser());
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({extended: true}));
app.use(express.static(path.join(__dirname, 'app')));   //设置静态目录

app.use(function (req, res, next) {
    res.sendFile(path.join(__dirname, '/app/index.html'));
    return next();
});

app.all('*', function (req, res, next) {
    res.header("Access-Control-Allow-Origin", "*");
    res.header("Access-Control-Allow-Headers", "X-Requested-With, Content-Type");
    res.header("Access-Control-Allow-Methods", "PUT,POST,GET,DELETE,OPTIONS");
    res.header("X-Powered-By", ' 3.2.1');
    res.header("Content-Type", "application/json;charset=utf-8");
    return next();
});

io.on('connection', function (socket) {
    console.log("some");
})

//接受子进程发送数据
wk.on('message', function (msg) {
    //所有客户端接收数据
    console.log(msg);
    io.sockets.emit('test', msg)
});

app.post('/', function (req, res) {
    //将数据传给子进程
    wk.send(req.body);
    //返回客户端数据
    return res.send(
        {
            status: 0,
            de: '123'
        }
    );
});

app.post('/chart', function (req, res) {
    var id, item, table, mapTable, mapQuery, sql;
    switch (req.body.day) {
        'xxxxxxx'
    }
    switch (req.body.area) {
        'xxxxxxxx'
    }
    switch (req.body.item) {
        'xxxxxxxxx'
    }

    sql = 'select time, id, ' + item + ' from ' + table + ', ' + mapTable + ' where  moistureinfo.ID = sensormap.sensorid and loggerid = ' + id;

    connection.query(sql, function (err, rows) {
       if(err){
           console.error(err);
           return ;
       }
        res.send({
            status: 0,
            data: rows
        });
    })
});

如果我把res.send()放在connection.query()后面，是能向前台返回数据的