# Flow vs. Typescript

## Flow

_Built by Facebook_

_Stats for past month: 0 merged PRs, 30 Proposed PRs, 50 closed issues, 135 new issues, SO Questions ~0.9k_

Used by: Facebook...

> PROS
- 👍  Built by Facebook, just like React – _in practice, this doesn’t seem to provide a marked advantage for flow_
- 👍  Babel integration
- 👍  No _real_ learning curve
- 👍  Inference of types
- 👍  support for JSX

> CONS
- 👎  No “out-of-the-box” integration with any editor
- 👎  Poor error messaging
- 👎  Poor tooling (Getting Flow to run is not easy)
- 👎  bug with JSX spread operator ( `<Component {...props} />` )
- 👎  Does it detect unused vars? “No. Use a linter.” -A Flow developer response :(
- 👎  non-native watch mode
- 👎  `.js` files could be flow, could be normal JS – not obvious at a glance

> MEHs
- 👍 /👎  Flow is highly interested in creating a provably correct type system
- 👍 /👎  ~400 typed third-party libraries


## Typescript

_Built by Microsoft_

_Stats for past month: 215 merged PRs, 46 Proposed PRs, 440 closed issues, 215 new issues, SO Questions ~38k_

Used by: Lyft, Microsoft, Google, Asana

> PROS
- 👍  Out-of-the-box, first class VSCode integration
- 👍  Easier to setup
- 👍  Used by default in Angular2
- 👍  Helpful error messaging
- 👍  `.ts` files easy to distinguish between `.js` files
- 👍  (as of 2.3) ability to type-check non-TS files with `--checkJs`
- 👍  support for JSX
- 👍  Babel integration
- 👍  Native watch-mode (file specific)
- 👍👍👍  ~3,000 third-party libraries

> CONS
- 👎  Possible increased language complexity --> learning curve
- 👎  In my experience there are frequent breaking changes from version to version (not sure if this is still a problem)

> MEHs
- 👍/👎  More frequent releases, this can introduce breaking changes
- 👍/👎  Typescript is not interested in creating a “provably correct” type system – instead tries to strike balance between correctness and productivity


### Links

- https://engineering.tumblr.com/post/165261504692/flow-and-typescript
- https://jan.varwig.org/2017/02/15/flow-vs-typescript.html
- https://eng.lyft.com/typescript-at-lyft-64f0702346ea
- https://medium.com/@ckoster22/migrating-from-flow-to-typescript-b065796797db
- https://www.reaktor.com/blog/refactoring-30000-lines-js-types/