Why not add the exclusion to the .arclint file as well, so it's all in a single place ?

The general pattern is that arclint includes the top level directory. app-specific exclusions are handled by the respective .eslintignore files in each dir. What we choose to ignore is often dynamic and imo best handled at the level of the specific app / subdir in the monorepo.

This is a weird case in Cashtab where we have a JS app, initially a Create React App, which uses nodejs scripts to build the react app -- now ejected from Create React App (to customize those scripts for polyfills related to crypto), and (partially) converted to typescript.

I'm still figuring out some of the ts settings and how they interact with things like jest and eslint. In this case, the ts linter is using tsconfig.json to decide what to lint. because tsconfig.json is not including the build dir (sensibly), build is not being picked up by the eslintignore.

This kind of weird edge case is the sort of thing that is best handled at the cashtab/ level and not arc lint. Going forward, will probably find a way to modify tsconfig.json so that eslintignore works. However, this tsconfig.json is still being updated/changed as I convert more of Cashtab to ts. So for now, excluding these dirs is probably the best way to fix the linter and get the benefit of CI linting.