[MarcusBrutus]

I 've used both. Flow wins hands down:

(1) It is less intrusive. Flow's type system is built on annotations which are simply stripped away at runtime (as opposed to writing in another - albeit similar - language that gets transpiled to JS. Sure the transformations are always "file-local" as TS folks say but still it is another, totally unecessary level of abstraction and transpilation.

(2) Flow's type system is at least equally and quite possibly much more expressive and nuanced than TS.

(3) Flow annotations can be applied in a much more piece-meal way than TS.

(4) zero issues integrating with 3rd party libraries (because of 1) - and that includes the ability to type-check said 3rd party libraries (which is a breeze with flow-typed)

(5) To take advantage of TS you have to write JS code in the "JS with classes" style (a.k.a "JS for C# coders"). In contrast Flow allows you to take advantage of its type checking while coding: (a) in idiomatic JS (prototypes) (b) in JS with classes (c) any combination and mix of the two approaches. Flow feels a lot less opinioned as to your JS programming style.

(6) Flow it is not coming from microsoft (but hey, don't you know we've changed, please give us another chance and we won't fuck you over this time, please we love you so). TS team is trying very hard (perhaps too hard) to distance themselves from the negative brand value of microsoft

(7) Flow is not a blueprint for the Mother of all embrace-extend-extigunish strategies (see #6).

[nycdotnet]

I have not used Flow, but there's a few misconceptions about TypeScript in the above comment.

- There's an implication that Flow is vastly different than TypeScript because its annotations are stripped away - that's how TypeScript works too.

- There's an implication that TypeScript has some sort of unusual syntax - Modern JavaScript (ES6+) and TypeScript code is identical (except for the addition of type annotations where desired).

- Flow may have a nuanced and expressive type system - I don't know - but TypeScript attempts to accurately model the type system of JavaScript.

- TypeScript can be used in a very loose mode where it type-checks plain .js files with no annotations at all, all the way up to a very strict mode where it must know the type of everything (which you can set to `any` on a per-variable basis if you don't care).

- TypeScript is JavaScript, so it has no issues integrating with any JavaScript library. If you want perfect strong typing of a given version of a given JS library, someone has to do that work and this is where some people do struggle. Microsoft has put forth a lot of effort to help here recently. Flow-type may be a great resource (I don't know), but I can scroll its list of supported libraries on its repo on GitHub. https://github.com/flowtype/flow-typed/tree/master/definitio... . The DefinitelyTyped repo breaks GitHub with the number of libraries it has definitions for: https://github.com/DefinitelyTyped/DefinitelyTyped/tree/mast... (nearly 2000 entries are omitted).

- Item 5 is flatly false. For example, TypeScript is itself written in TypeScript and there is not a class in the entire code base - it is 100% idiomatic functional JS. By contrast, Flow is not self-hosted.

- Regarding item 6, TypeScript is the poster child for the new Microsoft. It has been open source (Apache 2.0) and cross platform from its original release in 2012. And regardless of what happens with TypeScript (I forsee it tracking ECMAScript standards for the next several years at a bare minimum), the emitted JS is basically identical to your TypeScript code (minus the annotations) so worst case scenario if Microsoft somehow magically deleted all copies of TypeScript in the world, you'd just keep going with the ES6+ code you have.

[Rapzid]

I'll go out on a limb though and posit that classes are now idiomatic JS when doing OO. Shops are not cutting modern JS with objects built up via direct prototype manipulation.

[Etzos]

Thank you. I wanted to say almost exactly the same thing, but you worded it much better than I could have.

Point number one in particular bothered me since both languages are transpiled (in both cases that just means types stripped). However, I think MarcusBrutus was talking about Flow type comments which can be added to Javascript files to get type checking, but which will not need to be transpiled at all as all nonstandard stuff is in the comments.

[_f8tq]

Flow is not a transpiler, there is no concept of "emitting code", only of stripping away the annotations. In contrast, TypeScript is a transpiler. TypeScript being a transpiler means what gets executed at runtime is not what you wrote but some other sources emitted by the TypeScript transpiler. You say that presently (apparently due to the delta between ES6+ and TypeScript being currently very small) what is emitted is not materially (for some definition of "materially") different from the input sources. Even if that's true, this is simply a transient situation that may not hold next year (we're still in the "embrace" stage after all) and certainly didn't hold when I looked at TypeScript circa 2013.

Otherwise, can you please explain why bother implementing a transpiler if the emitted sources are identical to the input sources minus the annotations?

Even the names should give you a hint: "TypeScript" is a language name (the microsoft version of ECMAScript more specifically). "Flow" is not. That's why it's called "Flow" and not "FlowScript".

[alkonaut]

> It is less intrusive. Flow's type system is built on annotations which are simply stripped away at runtime

> as opposed to writing in another - albeit similar - language that gets transpiled to JS

How is a nicely typed bit of flow-code different from the equivalent TS? If you make actual use of the type system in either of Flow or TS you should end up with code that looks a lot like Java or C#, with generic types and functions etc.

It's nice that you can do it gradually in flow for an existing codebase, but it's still no difference once properly applied in full.

[jnordwick]

the whole "MS is evil" thing is so played out.