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 modes.

Classifying Files

Lazy mode tries to classify 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

There are two ways which Flow can use to tell which files the user cares about.

  1. IDE lazy mode. The IDE tells Flow which files have been opened and closed via flow lsp. Flow treats any file which has ever been opened since the Flow server started as focused.
  2. Filesystem lazy mode. Flow treats any file which has changed on the filesystem as focused. This mode is easier to use from the commandline, but a rebase can make every file appear focused.
  3. Watchman lazy mode. When starting up, Flow treats any file as focused if it has changed since the mergebase with master (the common ancestor of the current commit and the master branch). Any subsequent file that changes is also focused. After a rebase which changes the mergebase, Flow may restart the server if it estimates that a restart is faster than a recheck.

Using IDE Lazy Mode

To start a Flow server in IDE lazy mode, you run

flow server --lazy-mode ide

Alternatively, you can set the lazy mode from the .flowconfig.

The IDE needs to integrate with flow lsp to tell Flow which files are open.

Using Filesystem Lazy Mode

To start a Flow server in Filesystem lazy mode, you run

flow server --lazy-mode fs

Alternatively, you can set the lazy mode from the .flowconfig.

Using Watchman Lazy Mode

Watchman lazy mode has a few additional requirements.

  1. The Flow root must be within a Mercurial repository.
  2. Watchman must be installed and the watchman binary must be in the PATH.

To start a Flow server in Watchman lazy mode, you run

flow server --lazy-mode Watchman

Alternatively, you can set the lazy mode from the .flowconfig.

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

Was this guide helpful? Let us know by sending a message to @flowtype.