I lied a bit in the title: this works for any static analysis tool. So if you're a RuboCop, Pylint, or [insert awesome static analysis util here] user, this solution has you covered. The impetus for this post was ESLint, though, so I'll use that for the sake of a specific example.

I found myself in the unenviable position of trying to introduce a new linter rule. The reason this was such a pain was because there were over 4,000 violations in our codebase, and this particular rule didn't have an automated fix available. The manual fix wasn't just a matter of style, either — it would require regression-testing every module it touched. That's out of the question. Ignoring the error was impossible, too, because our CI build runs eslint against all of our frontend code. And making it a warning would not sufficiently incentivize the team to fix the underlying issues this rule is supposed to police. What to do?

Transitional config

First, I set up a "transitional" config file:

# .eslintrc.transitional
extends: ./.eslintrc
  my-new-rule: error

Note that this extends our existing configuration, adding just one modification. To ensure that any text editor integrations register the rule so that it's visible, I also modified the main config:

# .eslintrc
# ...
  # ...
  my-new-rule: warn

Ok, so the groundwork is there. Next, I set up a new script in package.json:

  "scripts": {
    "lint:transitional": "eslint -c ./.eslintrc.transitional --no-error-on-unmatched-pattern"

Using git diff

Then, the secret sauce: in the same CI build step as where we run eslint against our entire frontend codebase, I added a step where we run lint:transitional only on files that have been changed in that particular branch. git to the rescue! One hiccup we ran into is that deleted file paths were being checked, which caused eslint to return an error code. Adding --no-error-on-unmatched-pattern resolved this issue.

# get a list of js/ts files that are different from master
# at the merge-base
DIFFED_FILES_TO_LINT=$(git diff master... --name-only -- "*.ts" "*.tsx" "*.js" "*.jsx")

# execute eslint with the "transitional" rules
# only on touched files.
yarn lint:transitional $DIFFED_FILES_TO_LINT

For more info on that git command, check out this explainshell. This StackOverflow answer is also illuminating.

What did that accomplish?

We can now introduce a painful but ultimately helpful new rule gradually. When a team touches a file with which they are familiar, paying down the technical debt pointed out by the new rule becomes a matter of necessity. Since (presumably) developers are only touching files that are relevant to the work they are shipping, the chance of a regression is lower than it would be if some intrepid but less-familiar engineer took it upon themselves to try to update the whole code base at once. The total number of violations will gradually decrement, and so too will the as-of-now unknown pain points that may be associated with fixing them.

Keep the conversation going

I really appreciate feedback from anyone and everyone who reads my posts, so please feel free to say hi at [email protected] and keep the conversation going.