Type Inference

Flow uses type inference to find bugs even without type annotations. It precisely tracks the types of variables as they flow through your program.

Realtime Feedback

Flow incrementally rechecks your changes as you work, preserving the fast feedback cycle of developing plain JavaScript.

What are the benefits of using Flow?

Catch errors early
Improve readability of code
Facilitate tooling

Example

Example w/ Flow

Flow is a server

>> flow status

>> flow check

Sometimes you’ll make a change that forces the Flow server to restart.

changes to your .flowconfig file

Flow reads your entire node_modules directory

>> flow stop

>> flow init

Third-party libraries

Props and State

Missing @flow annotations

Flow will ignore it by making its type any

Weak mode

/* @flow weak */

Type Annotations

Using flow-typed

npm install -g flow-typed

Primitives

boolean
number
string
null
void

any

The type is a supertype and subtype of all types.

mixed

is like a safe but somewhat annoying version of any

Optional types

T|void, for some type T

Maybe types

T|void|null, for some type T

Arrays

Tuples

Tuple types are a kind of array type particularly suited to describe finite, heterogeneous collections