Using NestInterceptor to capture requests and responses

#nestjs #express #requests #responses

We needed a provision to capture API requests and responses. This was so that our mobile team could get a summary of what the app was sending and receiving. Once captured we wrote them in a file in the last-in-first-seen order.

We eventually started off by implementing the Nest Middleware but then we realized a middleware can only receive requests. So we switched to Interceptors.

An interceptor has access to both requests and responses.

Here is the code.

class LoggerInterceptor implements NestInterceptor
{
    intercept(context: ExecutionContext, next: CallHandler): Observable<any>
    {
        let logFile = process.env.LOGGER_APP + ".log";
        let logPath = process.cwd() + "<path>" + logFile;

        let { url, method, headers, body } = context.switchToHttp().getRequest();

        if (url != "/logs/dev.log")
        {
            let logMessage = "\n\n=======================================\n\nTIME: " + Date().toString() + "\nMETHOD: " + JSON.stringify(method) + "\nURL: " + JSON.stringify(url) + "\nHEADERS: " + JSON.stringify(headers) + "\nBODY: " + JSON.stringify(body) + "\nRESPONSE: ";

            return next.handle().pipe(tap((data) => 
            {
                let responseBody = JSON.stringify(data);

                logMessage += responseBody;

                // Reading old data.
                logMessage += fs.readFileSync(logPath);

                fs.writeFile(logPath, logMessage, function (err, file) { });
            }));
        }
    }
}

A couple of points worth mentioning -

is the path to where your log files should be stored. We wanted the app team to have access to the logs so we kept them in a public folder.
We put up a check (!= "/logs/dev.log") to prevent logging access to the log file into the log file :D Lol!

Happy Programming!!!