Lazy Modes

Explanation of Flow's lazy mode

By default, the Flow server will typecheck all your code. This way it can answer questions like “are there any Flow errors anywhere in my code”. This is very useful for tooling, like a continuous integration hook which prevents code changes which introduce Flow errors.

However, sometimes a Flow user might not care about all the code. If they are editing a file foo.js, they might only want Flow to typecheck the subset of the repository needed to answer questions about foo.js. Since Flow would only check a smaller number of files, this would be faster. This is the motivation behind Flow’s lazy mode.

Classifying Files

Lazy mode classifes your code into four categories:

  1. Focused files. These are the files which the user cares about.
  2. Dependent files. These are the files which depend on the focused files. Changes to the focused files might cause type errors in the dependent files.
  3. Dependency files. These are the files which are needed in order to typecheck the focused or dependent files.
  4. Unchecked files. All other files.

Lazy mode will still find all the JavaScript files and parse them. But it won’t typecheck the unchecked files.

Choosing Focused Files

Focused files are all of the files that have changed in one of the following ways.

Flow will focus files when they change on disk, using Flow’s built-in file watcher (“dfind”) or Watchman.

So, all files that change while Flow is running will be focused. But what about files that change when Flow is not running? If you’re using Git or Mercurial, Flow will ask it for all of the files that have changed since the mergebase with “master” (the common ancestor of the current commit and the master branch).

If you’re not using “master” (e.g. “main” instead), you can change this with the file_watcher.mergebase_with config. If you’re working from a clone, you might want to set this to “origin/master” (for Git), which will focus all files that have changed locally, even if you commit to your local “master” branch.

The net result is that Flow will find the same errors in lazy mode as in a full check, so long as there are no errors upstream. For example, if your CI ensures that there are no errors in “master,” then it’s redundant for Flow to check all of the unchanged files for errors that can’t exist.

Using Lazy Mode

To enable lazy mode, set lazy_mode=true in the .flowconfig.

To start a Flow server in lazy mode manually, run

flow server --lazy-mode true

Forcing Flow to Treat a File as Focused

You can force Flow to treat one or more files as focused from the CLI.

flow force-recheck --focus ./path/to/A.js /path/to/B.js

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