How to debug Next.js core?

[jensmeindertsma]

I'm working on a pull request that changes the Next.js core. I'd like to be able to inspect the value of different variables at runtime. A console.log inside for example next.js\packages\next\build\webpack\config\blocks\css\loaders\client.ts does not work, and I'd like to use the VSCode debugger and set breakpoints.

I've noticed that the root of the project does contain a .vscode/launch.json file (this suggests debugging is possible). Running yarn dev does not work, I suppose because it builds the code and doesn't execute it?

I've read the contributing guide and the debugging guide ( that is meant for bugs inside my Next.js app, not the core? ), but the contributing guide doesn't say anything about debugging the core.

Any help with this would be appreciated. Thanks in advance!

[jamesmosier]

Have you tried yarn debug from within the root directory?

https://github.com/zeit/next.js/blob/9fb2432b14ed85889b892e5ba024fb07a204faf7/package.json#L36

I haven't tried this myself, but seems like a good starting point.

[jensmeindertsma]

Yeah I tried this but the build instantly fails because there is no pages directory. I tried running the Launch app development debugger to run an app ( so that there are pages), but it only triggers a few of my breakpoints. Can you help me with solving the no pages issue?

[timneutkens]

If you use vscode you can boot up one of the integration test apps using the debugger inside of vscode.
In other cases you can run yarn debug test/integration/basic (or any other directory with a Next.js app in the monorepo, example: yarn debug test/integration/production (boots up in development mode) or yarn debug build test/integration/production debug === next (the Next.js cli)

[jensmeindertsma]

Thanks so much! I got it all working after removing the .next directory from test/integration/basic that had apparently cached the Babel plugins because the breakpoints inside the Babel plugin files were not triggering during compilation. It's a bit random when you don't remove the .next folder. Sometimes they get triggered, other times they don't.