There are some peculiarities.
You can open port on application start and reconnect on port close or open port on each request. It defines how work with data flow. If you send request to port then answer can contain data of previous requests (more than one). You can ignore this problem (if answer is short and request interval is enough large) or send request with assign id and search answer with this id.

SerialPort.list(function (err, ports) {
    ports.forEach(function(port) {
        console.log(port.comName, port.pnpId, port.manufacturer); // or console.log(port)
    });
});

router.get('/', function(req, res){
    function sendData(code, msg) {
        res.statusCode = 500;
        res.write(msg);
        console.log(msg);   
    }

    var port = new SerialPort("COM5", {
        baudRate: 38400
    });

    port.on('open', function() {
        port.write(Buffer.from('status1', 'ascii'), function(err) {
            if (err) 
                return sendData(500, err.message);

            console.log('message written');
        });
    });

    var buffer = '';
    port.on('data', function(chunk) {
        buffer += chunk;
        var answers = buffer.split(/\r?\n/); \\ Split data by new line character or smth-else
        buffer = answers.pop(); \\ Store unfinished data

        if (answer.length > 0) 
            sendData(200, answer[0]);
    });

    port.on('error', function(err) {
        sendData(500, err.message);
    });
});

module.exports = router;

I cannot test this end to end (no arduino anywhere near), but since serialport seems to implement a Readable stream, I'd try using scramjet like this:

const {StringStream} = require('scramjet');

serialport.on('open', () => console.log('open');
serialport.pipe(new StringStream) // pipe the stream to scramjet StringStream
    .lines('\n')                  // split per line
    .each(                        // send message per every line
        data => io.sockets.emit('message',data)
    );

Scramjet would sort the readline issue for you.

I found the answer myself!
I was using the wrong baudRate, and also needed to stringify the data being sent as a JSON string

Author of node-serialport. I have tracked down the issue and it is due to a compilation issue with IOWatcher in node.js. I have revised the strategy for reading from the serial port and it now should function as designed in all cases. Please ensure you are using node-serialport 0.2.6 and greater.

Now go out and build JS controlled robots!!!