Debugging a Frontity App is no different than debugging any other node.js app or any other Isomorphic React App.
Frontity works with Isomorphic React Apps which means that the same React app will be used to generate HTML from the server and to hydrate/routing your app from the Client Side
Because of this there will be parts of your Frontity project that will be executed in the server and some others in the client (in the browser) depending on the use case
So, you can debug both server side and client side executions
to debug Server Side executions
For that you can use for example the extensión and steps suggested by @Chris1337
You can also use console.log but the parts executed in the server will output in the terminal
to debug Client Side executions
Here, you can also use console.log but the parts executed in the client side will be displayed in the browsers console
Another option is to use the sentence `debugger`` which will stop the execution of the client side code at a specific point in your browser’s Developer Tools
To have more info about your React Components used in your Frontity App you could also use React Developer Tools
Just to add more info regarding this topic, we have on our roadmap a plan to create a frontity admin UI that will include a dev tool that would be similar to redux DevTools or overmind DevTools
This should make debugging Frontity apps more transparent!