flow

Type check project

$ flow check

$ flow server

$ flow check-contents < [file.js]

$ flow init

$ flow type-at-pos [file.js] [10] [5]

flow [command] [options]

flow is a static type checker for JavaScript from Facebook. It adds gradual typing through type annotations and inference, catching errors before runtime.
The tool runs as a background server for fast incremental checking. Type annotations use a superset of JavaScript syntax, stripped at build time.
Flow integrates with editors for real-time type checking and provides detailed error messages with suggested fixes.

COMMAND

Operation: check, server, status, init, etc.

Type check all files.

Start background server.

Show server status and errors.

Create .flowconfig file.

Stop background server.

Get type at position.

Display help information.

.flowconfig

Configuration file controlling which files Flow checks, library definitions, linting rules, and options for type checking behavior.

Requires .flowconfig file. Not compatible with all JavaScript patterns. Build step needed to strip annotations.

Flow was created at Facebook (now Meta) in 2014 to add type safety to their JavaScript codebase. It competed with TypeScript, offering a different approach to gradual typing.

tsc(1), eslint(1), node(1)