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There appears to be a real movement to move from Flow to typescript. Is Flow dying?

I tried Flow and TypeScript out about half a year ago in a client project and Flow was kind of painful and difficult to use, the updates, bug fixes and new features weren't coming as often as TypeScript's were, and maybe 70% of people I talked to were using TypeScript instead of Flow. Since then it's more like 90% now, and the TS team seems to be adding and announcing features faster than before, whereas I haven't heard about any new features in Flow. Maybe they're there, I'm just saying I haven't heard about them. This is all anecdotal but from what I've heard, it's very common. This is a big reason I chose TypeScript in Autumn. Also, VS Code has become the de facto free editor of JavaScript projects with a full set of professional-grade IDE features across the board, and VS Code has first-class support for TypeScript but only plugin-level support for Flow. Since Monaco is simply extracted from VS Code, that means I got all VS Code's IDE features for free when I embedded Monaco into Autumn, all I had to do was set the language to "typescript" and it just works. All these free and serious benefits of TypeScript's ecosystem kind of point to this conclusion that I've seen on HN and elsewhere over and over, that TypeScript has already won and Flow is on its way out.

Facebook (excuse me, bunch of FB employees from the Flow team) says no: https://github.com/facebook/flow/issues/7365#issuecomment-45.... Of course, that's exactly what I'd say if I was in charge of a dying project.

Edit: but, their main codebase is in Flow, and that sounds like a mess to migrate, so I wouldn't be _too_ worried. It might slow down, but I doubt it will become unmaintained anytime before Facebook gets sued out of business :-P

I recalled when Microsoft employees and MVPs denied when Silverlight was dying. :-( I think the TypeScript momentum is too strong now.

I think Flow's in more direct trouble, but I wouldn't announce victory for TypeScript just yet. With wasm around the corner, things may (or may not, shrug) change significantly.

Even with wasm, much as I'm looking forward to the Great JS Purge personally, it's not going to happen anytime soon - too much existing code and devices. JS will remain necessary on the front-end for at least another decade, and probably beyond that. So TS will still be necessary to make the pill less bitter.

Both yarn and jest have announced they're switching to TypeScript. A corporate like Facebook would never allow that unless there was an internal shift in direction.

I'd consider flow dead.

> A corporate like Facebook would never allow that unless there was an internal shift in direction.

Sure they would. Flow has nothing to do with Facebook's business strategy, or its marketing campaigns, or even its corporate strategy. I'd be surprised if anyone on VP or higher even knows what it is.

Insofar as "NIH" wins at big companies, it's when there's a business motive to promote a certain technology or FUD about what other technologies might exist.

But engineering team A deciding not to use engineering team B's tools despite working at the same company? Happens all the time.

Engineering VPs very much know about it.

The writing has been on the wall for a very [1] very [2] long [3] time. Facebook and some consumers of their ecosystem have been holding the fort, but it was inevitable something like this would happen.

1: https://npm-stat.com/charts.html?package=babel-core&package=...

2: https://trends.google.com/trends/explore?date=today%205-y&q=...

3: https://developers.slashdot.org/story/18/11/25/017227/micros...

Well, TypeScript at least is growing enormously. Whether Flow dies or not is up to its authors, but I think TypeScript is pretty much unstoppable today.

I maintain PayPal's cross domain suite of libraries [0] and I'm fully intending to drop Flow for TS. The single thing I'm waiting for is https://github.com/Microsoft/TypeScript/issues/21699 since we use a fair amount of custom jsx rendering [1]. That's the one main thing (for me) that Flow is way stronger at right now.

[0]: https://medium.com/@bluepnume/introducing-paypals-open-sourc...

[1]: https://medium.com/@bluepnume/jsx-is-a-stellar-invention-eve...

There have been several projects that were flow based that have moved to TS. I think the real litmus test will be if React eventually either includes TS types or is rewritten in TS.

Disclaimer: I work with TypeScript professionally.

I'm pretty in tune with what's going on around React, and I don't see that one ever happening.

The React team is _very_ busy already with work around Hooks, Concurrent Mode, and Suspense. There's no way they're going to pause development on implementing all these major chunks of functionality just to rewrite from one type system to another.

I've seen Dan express some frustration with Flow's pace of development on Twitter a couple times, but beyond that, no indications whatsoever that React would be converted to TS. In the entirely hypothetical scenario that React _did_ get rewritten to another language, I have to assume it would be something like ReasonML (which was created by Jordan Walke, the original creator of React).

One of the benefits to TypeScript is that the codebase wouldn't need to be rewritten.

A declaration file(s) could be included. There they could just declare types for all classes, methods, constants, etc. Similar to C header files.

(This is how the DefinitelyTyped repository handles typings for untyped source repositories: https://github.com/DefinitelyTyped/DefinitelyTyped)


JavaScript: app.js

    function app(arg1, arg2, arg3) {
       // does something
       return {
          key1: someStringValue,
          key2: someNumberValue,
TypeScript: app.d.ts

    declare type AppReturnValue = { key1: string, key2: number };
    declare function app(arg1: string, arg2: string[], arg3: boolean): AppReturnValue;

Given that React is already heavily invested in Flow types, there would be _some_ form of rewrite.

And like I said, Flow is providing sufficient benefit for the React team right now, and their focus is on expanding React's capabilities. Changing type systems is not on their radar as far as I know.

Oh— can you point me at a file in particular? I had a look around the codebase and didn't see anything but plain JS. Might have been looking in the wrong places.

And I certainly get your point. However I wonder if they'd consider community-contributed TypeScript declaration files to the official repo as they wouldn't cause conflicts with the Flow system—

Flow types are _everywhere_ in the codebase.

To pick a specific example, here's the file that implements the core logic for the new Hooks feature:


As for the TS typings, there's been lots of agitation from people asking them to be officially included and shipped with React. But, again, the React devs themselves aren't TS users (that I know of), and so they don't have the expertise to write and maintain those typings. Better that they be left over in DefinitelyTyped for the community to maintain.

(I'm a Redux maintainer, and I feel exactly the same way about the typings for React-Redux. I don't have any actual TS experience myself yet, and I couldn't do anything useful in regards to the React-Redux typings. Plus, I've got far too much else on my plate to worry about those.)

I'm curious as to this trend as well. I've read the justifications for it in each case and in this particular one I don't get it.

Isn't Flow more concerned with soundness than intellisense? Has TypeScript caught up with Flow in this regard?

Perhaps it's because I prefer to do my work in strongly typed, pure FP languages where I can but I work professionally with JS and have been investing in Flow there for over a year now. While Flow is still not exactly great it can at least mimic exhaustive pattern matching and catches most unsafe type errors without getting in the way of common JS patterns too much.

The reason the yarn maintainers are giving is because they want more contributors? What advantage will that have if TypeScript isn't catching the errors you used to be able to catch or don't know about yet?

Curious to know if it's worth migrating over to TS without sacrificing anything other than the minor inconvenience.

> Has TypeScript caught up with flow in this regard

I think the answer is: it's very close. It's still a little behind flow on soundness, but it's now close enough (if you enable strict mode, which you should!) that you're unlikely to notice the difference.

The TS type system is very impressive. It can't do everything that the functional languages can do, but it can do some things that they can't, and importantly it's still improving rapidly.

You may be interested in the roadmap/changelog page: https://github.com/Microsoft/TypeScript/wiki/Roadmap

Does anyone know of any "third choice" around typing JavaScript? I would love to add types to my code, but I want to write "real" JavaScript: so the code I input is the code that is executed by the browser. I just want the compile step to strip away the type annotations.

There was initially talk of Flow using comments to actually work without touching the source code at all, but I don't think anything came of that... is there anything else on the horizon?

[Edit: Nevermind: I went looking for the github issue about adding types as comments, and it turns out it's already supported by flow: https://flow.org/en/docs/types/comments/ - is there anything like this for TypeScript?]

Yes, TypeScript can do type checking on regular Javascript (annotated with comments) as well: https://www.typescriptlang.org/docs/handbook/type-checking-j...

> I would love to add types to my code, but I want to write "real" JavaScript: so the code I input is the code that is executed by the browser. I just want the compile step to strip away the type annotations.

That's what Typescript is. Type annotations don't change your code, they're stripped away by babel at compile time. In fact, by default, tsc does compile TS code which fails typechecking, into valid JS code (which will likely error when you use it at least in some circumstances, but can run just fine).

Typescript is a clear superset of JS, so there's no actual logical changes or even syntactical changes done by the compiler. However, you may be confused by the often-used "compilation target" features of babel (and tsc), which is that you may write ES2018 code, target ES5, and have your ES2018 syntactical sugar be turned into ES5-compatible code. This is entirely opt-in, and not related to typescript (other than the fact that Typescript is always compatible with the latest ES spec, so you can use any legal ES2018 syntax in it).

Hope that clears it up…

This is where I see a big difference between Flow / TS.

With Flow, the type annotations are just stripped away, none of your Flow code affects runtime code.

This is not so with TS since you have things like Enums, which will be compiled into objects and are part of your runtime code.

If you write type annotations in TS, they're stripped away.

If you write type annotations in Flow, they're stripped away.

There's no difference. TS has some additional features which are purely optional that don't get purely stripped out, but you're not mandated to use them.

There is no difference if you limit the use of the tool to not use these features.

In my eyes, it's a philosophical difference between the two, Flow can be more easily integrated into an existing codebase by just adding //@flow at the top the file and has no features which can affect runtime code. Whereas TypeScript tries to be a different language altogether that uses a new file extension, adds new features, and has its own compiler.

When you can implement a tool like this using TS, let me know https://github.com/flowtype/flow-remove-types

Typescript doesn't try to be a different language. It tries to be an exact superset of JS: JS with annotations. And you can absolutely implement what you linked, it exists and it's called tsc: the "compiler" you're somehow upset Typescript has (this + checking types is all it does!). Babel does it natively.

Enums are a fair point, those aren't in the ES specs (though I suspect at some point they will be). However, they're an incredible addition and they really are just syntactic sugar for a more complex type of object.

BTW, typescript supports jsdoc-style annotations, and --allowjs even lets it typecheck javascript code. It also supports more, because nobody actually only wants those things; they're not that great on their own.

Not sure where you got the idea that I'm upset that TS has a compiler, I'm just pointing out my perspective on the differences between the two tools. I use both on a daily basis.

You might not think the differences is a big deal, but affecting runtime code is a pretty major line to cross. Not that there is anything inherently wrong with that, but at that point it becomes a different tool, in my opinion.

No I get you, being able to map the exact code is important. But I think what's happening is you're confused about the ES20XX translation layers. Those aren't at the typescript level, they're at the Babel level … it just so happens that tsc supports that bit, but it's my understanding that this will be going away at some point (eg. typescript will move to only doing the typechecking, and leaving babel to only do the compiling).

enums are a tiny, extremely useful and extremely optional part of the language and they don't warrant this label of "philosophical difference", IMO.

I think Babel does excatly that if only enable the typescript-preset.

Is there any compiler option or linter config that prevents the use of these extra features?

tslint has no-namespace and no-enum to disallow the two things that create new runtime code

TypeScript works with JSDoc annotations: https://github.com/Microsoft/TypeScript/wiki/JsDoc-support-i...

Can confirm, have sometimes added /* * @type Foo */ above a type and VS Code assumed that type was Foo everywhere else in my project, as if I typed `foo: Foo` directly. Didn't even have TypeScript in the project itself, purely an IDE feature. Extremely well thought out and useful ecosystem.

If you're using Babel for compatibility with particular browsers then that can already handle TypeScript. What distinction are you drawing between a compile step that "strips away the type annotations" and one that does something else - what else is it that you consider compiling TypeScript to include?

I have to ask what you're trying to achieve here. Moving types into comments just gives people who build without your typechecker the chance to break your code.

The comment stripping is optional (just for saving bytes on download). The goal is moving away from Babel, and any other unnecessary transpiling steps. Now that JavaScript has (a semi-working) module system, I find my projects having fewer and fewer dot files and far fewer dependencies.

If I can have a folder of plain JS that I know will work in a browser 20 years from now without having to resurrect an ancient/abandoned toolchain, then I'll do that!

Are you gonna hardcode all the HTML and CSS as well? I would expect resurrecting a 100% compatible toolchain for a mainstream source format 20 years from now will be easier than resurrecting a 100% compatible browser. Especially if that format is a stricter/less ambiguous one like typescript.

Not exactly what you're asking, but a "third choice" in general to typed ECMAscript has been ActionScript. A quick google found a transpiler project, but it looks to be abandoned: https://github.com/Cleod9/as3js

Still, I once did Flex development (would compile to a SWF file, or with Adobe Air to a native executable). It was a pleasure -- so long as you used the Flex/Flash Builder IDE (which was itself built on top of Eclipse). Trying to develop in vim was harder, though mainly because of the MXML part, but I still liked MXML better than HTML. Later at the same job I did Node and found that pretty enjoyable too, plus I could use vim all the time.

Tooling is always a concern with a language. I think dynamically typed languages are partially successful because they let you get away with so much less tooling. Though not all dynamic languages are equal in the tooling they can trivially support, e.g. Common Lisp with Slime enables all the usual stuff (who calls x, who sets y, who specializes method z...) and you can use Slime from a variety of other tools.

Flow really shines when you have a large, untyped codebase and want to incrementally add types to it. If you're starting a new project (or rewriting one) TypeScript is the more sensible choice. By nature Flow is going to be used less and less over time.

For people now thinking well we have a large untyped codebase..damn.

You can turn typescripts type inference on on a regular js file.


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