used for hijacking jsonParser middleware and dumping the request/response bodies to text files; used with https://jvilk.com/MakeTypes/ to RE an API endpoint (black-box) and begin documenting/refactoring. especially helpful for monolith applications.

jsonParser_hijack_diff.js

const _jsonParser = express.json({ limit: '100mb' });
// hijack jsonParser to log JSON bodies
console.log('installing jsonParser hijack')
const jsonParser = (req, res, next) => {
    console.log('jsonParser called @', req.method, req.url);
    _jsonParser(req, res, (next_retval) => {
        // log if JSON request
        if (req.headers && req.headers['content-type'] && req.headers['content-type'].startsWith('application/json') && req.body) {
            let fname = `${humanizedISO8601DateTime()}-${req.url.replace(/\//g, '')}.json`.replace(/ /g, '_')
            let _path = path.join(__dirname, 'debugLogs', fname)
            console.log('logging to', _path)
            console.log('request was application/json', JSON.stringify(req.body).length, 'bytes')
            try {
                console.log('writing', _path)
                fs.writeFileSync(_path, JSON.stringify(req.body, null, 2))
                console.log('did write', _path)
            } catch (e) {
                console.log('failed to write', _path, e)
            }

            let resp_fname = `${humanizedISO8601DateTime()}-${req.url.replace(/\//g, '')}-response.json`.replace(/ /g, '_')
            let resp_path = path.join(__dirname, 'debugLogs', resp_fname)
            var oldWrite = res.write,
                oldEnd = res.end;

            var chunks = [];

            res.write = function (chunk) {
                chunks.push(chunk);
                return oldWrite.apply(res, arguments);
            };

            res.end = function (chunk) {
                if (chunk)
                    chunks.push(chunk);
                var body = Buffer.concat(chunks.map(chunk => typeof chunk === 'string' ? new Buffer(chunk) : chunk)).toString('utf8');
                // console.log(req.path, body);
                fs.writeFileSync(resp_path, body)

                oldEnd.apply(res, arguments);
            };
        }
        // console.log(req.headers)
        next(next_retval)
    })
}