Example of ExpressJS based server-side logging of client-side errors

client-error-handler.js

// need to load https://cdnjs.cloudflare.com/ajax/libs/UAParser.js/0.7.10/ua-parser.min.js or something similar
// to be used with window.onerror (e.g. window.onerror = logError);
const logError = (message, file, line, col, error) => {
    try {
        var parser = new UAParser();
        var userInfo = parser.getResult();
        var http = new XMLHttpRequest();
        var params = JSON.stringify({
            userInfo: {
                os: `${userInfo.os.name} ${userInfo.os.version}`,
                browser: `${userInfo.browser.name} ${userInfo.browser.version}`,
                device: userInfo.device.name ? `${userInfo.device.name} ${userInfo.device.version}` : 'unknown',
            },
            message,
            file,
            line,
            col,
            error
        });

        http.open("POST", "logservice", true);
        http.setRequestHeader("Content-type", "application/json;charset=UTF-8");
        http.send(params);
    } catch (e) {
        console.error("EPIC FAILURE, can't even log error: ", error);
    }
};

server-error-logger.js

app.post('/clienterrors', (req, res) => {
  var fs = require('fs');
  var body = req.body;
  var logStream =   fs.createWriteStream('/var/log/mas-react/client.log', {'flags': 'a'});
  req.body.time = new Date();
  logStream.write(JSON.stringify(body));
  logStream.end('\n');
  res.json({ logged: true });
});