A gist which shows how to run Morgan in Express with output in JSON

Express-Morgan-JSON.js

const express = require('express');
const app = express();
const morgan = require('morgan');
const port = 3000;

// Define your morgan logger to log JSON to your client
// The object here takes your keys and strings that use
// the morgan token format
app.use(morgan(format({
    response_time: ':response-time',
    remote_addr: ':remote-addr',
    remote_user: ':remote-user',
    status: ':status',
    url: ':url', // etc.; any non-standard tokens you would have to implement
})));

app.get('/', (req, res) => {
    res.send('Hello World!');
});

app.listen(port, () => {
    console.log(`Example app listening on port ${port}`);
});

function format(obj) {
    var keys = Object.keys(obj);
    var token = /^:([-\w]{2,})(?:\[([^\]]+)\])?$/;
    return function (tokens, req, res) {
        var data = {};
        for (var i = 0; i < keys.length; i++) {
            var key = keys[i];
            var val = token.exec(obj[key]);
            data[key] = val !== null ? tokens[val[1]](req, res, val[2]) : obj[key];
        }
        console.log(JSON.stringify(data, null, 4));
    };
}