Do you have some source files that are somehow exempt from bugs and would be a waste of the linter's time?

Probably not, but it's a trick question: if you try to look for exceptions to the rule, you have already wasted so much time that running a linter on all files would be faster.

>Do you have some source files that are somehow exempt from bugs and would be a waste of the linter's time?

Every file not touched in any given diff

What if the diff adds a new linter rule, should we only run it on the linter config file?

What if the linter uses more context than a single file, a type-checker for example or even just checking the correct number of arguments (regardless of type) are passed to an imported function - or that that symbol is indeed even callable? Should we only run the linter on the caller's file, or the callee's, when they haven't both changed?

Run the linter on the code base then, when you make the change? Not every check-in on the off chance a rule changed. Or, add some logic that the CI runs it against the whole code base only when the rules changed, otherwise just the relevant files to the commit/pr

Also, ESLint doesn't do type checking. That's typescripts job, and apparently typescripts runtime isn't an issue.

If a different (unchanged) file depends on the one you changed, you could have changed the API in a way that makes the unchanged file unacceptable to your linter.

If I change a function signature, then my code is fine - but all the other files which import and use my function will break

That's a job for TypeScript, not eslint.

Linter rules can rely on the type system

What eslint rule would apply to the caller of a function after that function's signature changes that wouldn't also be picked up by TypeScript?

In particular, the call site itself hasn't changed, as this thread assumes the linter is only run on changed files

Anamexis has a couple of examples in this response: https://news.ycombinator.com/item?id=38655101

Yes, because you lint everything in CI. Otherwise, linter warnings will start creeping into your codebase immediately, and the tool becomes much less useful.

I think if my CI was taking 45 mins to lint I'd look at linting only the files changed since the previous build instead of splitting it across 40+ workers. Or writing a new linter in Rust.

But I'm generally working in a (human & financially) resource-constrained environment.

Typescript lints are type-aware so you can’t just lint changed files, you have to relint the entire codebase to check if any type changes have impacted the unchanged code.

Wouldn't an issue with a type change be caught at typescript compile/check steps?

I'm not aware of eslint rules which would complain about some other untouched file if types have changed in ways such that the program still compiles

A few examples of typescript-eslint rules that could fail when a type in another file is changed:

https://typescript-eslint.io/rules/await-thenable/

https://typescript-eslint.io/rules/no-for-in-array

https://typescript-eslint.io/rules/no-duplicate-type-constit...

Is there no incremental lint mode? When developing you need that for instant feedback, same mechanism should work for CI.

One problem is that a change in a.js may trigger a new error in b.js.

ESLint could also cache things fairly trivially:

  hash = hash_file_contents()
  if previously_seen_hashes.contains(hash)
      report_previous_results()
  else
      run_lint_and_cache_results()
  end

maybe that already exists. But that has the same problem.

When you've got enough hardware to throw at it, then "just run it on the full code" is the safest.

I thought it would be obvious that in large codebases you only lint changed files in CI

Would not you lint only on files that changed?

I'm not sure if Eslint has this, but there could be cross-file lints (eg. unused variables). If some file changes, you may need to relint dependencies and dependent files. This could recursively trickle.

I'm not sure if Eslint does this either, but indices or some incremental static analysis sounds like it could help the linter minimize rechecks or use previous state.

You can tell eslint about globals in it's config. But if you're using variables that arn't declared in the file somehow, that might be an issue you want to look at in general. That's a potential foot gun a linter should be balking at.

if you have one file that every single file across the repo imports in a way and you make changes to that file, you might run the linter for the entire repo. But again, how likely is this scenario?

If the index or incremental static analysis object was designed well enough, I don't think you would need to lint every file, you would just need to look at files that consume that variable. Maybe you would look at every index?

I'm not sure how well this could scale across (600- 1000?) different lints though. I should look into static analysis a bit more.

As the sibling comment mentions, you may have lint rules that depend on checking for the existence of, or properties of, another file. A popular set of rules comes from https://www.npmjs.com/package/eslint-plugin-import which validates imports, prevents circular dependencies, etc

File A depends on File B. File B moves. File A is now wrong, because it is unchanged.

Static analysis != linter.