Flow (JS)

Long ago when Angular 2 evolved I had to decide between the new #Angular and TypeScript or React. I really love typing my code, but forced to use TypeScript was a bit too much. I prefer the new #ECMA standard and the evolution of the old and reliable JavaScript. So finding Flow (JS) was an incredible milestone in my career as a developer. Finally, I could use types in my code, and JavaScript with the new standard. I already had the experience of CoffeeScript, so TypeScript was not an option.

I use TypeScript because I tried both on a Meteor project, and found the quantity of errors it enabled us to catch and the simplification of code it allowed was higher than Flow (JS).

We use Relay because it's ability to generate Flow (JS) types based on given GraphQL fragments gives an immense amount of type safety. The relay-compiler helps us verify queries are valid before we build the app. Relay's ability to colocate queries with components, and compose many fragments, makes it easy to build a new component and get the data the component needs.

If you will start a project from scratch I recommend to use TypeScript. But, If you work with legacy projects written in JavaScript I recommend Flow (JS). Both tools have the same objective: reduce the bad code (which create illegible code, generate bugs e problems to maintenance). Flex helps you to avoid fall in bad codes, but TypeScript prevent you to c you to create bad codes. I believe cause this some JavaScript fans don't like TS, because TS block you to write some types o code. This is the fundamental difference between TS and Flow: Flow avoid problems, but no force. TS force you to prevent problems.

We currently use TypeScript at work. Previously we used Flow (JS) but it was sometimes really difficult to make the types work the way you want. Especially non-trivial types were problematic. And the IDE support wasn't good, Flow took too much resources and sometimes remain stuck and do not show errors (I use Visual Studio Code). With TypeScript we almost do not have these problems. IDE support is superb, working with types is much easier and typing system seems more mature and powerful. There are some downsides (like partion inheritance etc.), but TS team is still pushing it forward. So for me TypeScript is clear winner.

I use TypeScript because the tooling is more mature (the decision to discontinue TSLint in favor of moving all its checks to ESLint is a thoughtful and mature decision), there's a ton of examples and tutorials for it, and it just generally seems to be where the industry is headed. Flow (JS) is a fine tool, but it just hasn't seen the uptake that TS has, and as a result is lacking a lot of the nicer small things, like thorough Visual Studio Code integration, offered by TS.

We originally (in 2017) started rewriting our platform from JavaScript to Flow (JS) but found the library support for Flow was lacking. After switching gears to TypeScript we've never looked back. At this point we're finding that frontend and backend libraries are supporting TypeScript out of the box and where the support is missing that the commuity is typically got a solution in hand.

I use TypeScript because it isn't just about validating the types I'm expecting to receive though that is a huge part of it too. Flow (JS) seems to be a type system only. TypeScript also allows you to use the latest features of JavaScript while also providing the type checking. To be fair to Flow (JS), I have not used it, but likely wouldn't have due to the additional features I get from TypeScript.

I use TypeScript for Web Applications and for both frontend and backend because it has a lot of tooling around it and they really got the types and type safety right. Flow (JS) on the other hand lacks tooling and most of the times I scramble to find the right way of building my contracts in which TypeScript is very intuitive and natural. Additionally TypeScript is very similar to Java so your backend engineers and full stack engineers can work with it without much of context switch.

The only time I think Flow shines is (based on probably my outdated knowledge) Flow is/was the only option if you want/wanted to build a React Native application mainly because React Native transpiler at the time I was working with it would only work with flow.

Appitr Run JavaScript ES6 with React Native in the browser built on Monaco Editor, Browserify Webpack Sequelize npm Node.js ExpressJS Babel Flow Type ESLint MobX GraphQL