Mounting Loopback 4 Rest API in Express with Winston Logging

Download the working example from my Github repository here.

The Loopback team recently included the express-composition example for mounting the Loopback 4 Rest API in a regular Express application. See the full tutorial to do this on their official page here.

So, you can do something like this –

app.use("/path", loopbackApp.requestHandler);

This also allowed adding Express style middleware before passing control to the Loopback application. It is especially useful for adding Express middleware to check access tokens, authentication etc.

app.use("/path", checkAccessTokenMiddleware ,loopbackApp.requestHandler);

Loopback 4 does provide something called sequences for adding middleware logic, but that has to be written using the Loopback 4 directives.

I’ve modified theexpress-composition example, and added logging functionality using the Morgan and Winston logging libraries. I’ve tried to make it similar to an Express application that’s generated using the Express Generator which includes the Morgan HTTP logging middleware.

Getting the Winston logger stream to log the Morgan HTTP requests was a bit tricky with Typescript, but I got it to work in the end. The log files generated by Winston are also split based on the logging levels. So, separate files are created for error, info and exceptions.

You can download the working example from my Git repository here. Feel free to poke around, and enhance it based on your needs.

You’ll also find an errorlogging endpoint to log errors through a POST request. The errorlogger controller internally uses the Winston logger to log an error.

To make changes, start with the src/server.ts.It includes all the code for adding Express routes and middlewares.

Happy Hacking