Ask HN: Do you use Flow (JS type checker) in 2023?

https://news.ycombinator.com/item?id=34597060

by conaclos • 3 years ago

1 2 3 years ago

news.ycombinator.com

I am more and more thinking about switching from TS to Flow. I have always liked the strictness of Flow. The biggest disadvantage is the syntax incompatibility with TypeScript, especially declaration files.

Are you using Flow in 2023? What are your experiences with it?

acemarke• 3 years ago

Flow is effectively dead.

Yeah, there's still a team at Facebook developing it, but they've made it clear they don't care about any use cases or requests other than Facebook's own internal needs.

TS won, and the ecosystem has settled on it.

As one example, the current monthly downloads:

- TS: 37.3M

- Flow (`flow-bin`): 450K

lhorie• 3 years ago

We use it extensively but are migrating away from it at Uber.

Its strictness is a double edged sword, it often trips people up with checks that are pedantic but not fully sound. For example, TS will gladly accept that `if (typeof foo.bar === 'string') doStuff(foo.bar)` narrows down `foo.bar`'s type to string. Not so with flow... because Javascript supports getters and Flow's type system isn't strong enough to keep track of what is a getter and what isn't. So it will nag about it maybe not being a string, even when it isn't warranted (which is like 99.9% of the time this happens). There's also a long standing bug where await doesn't unwrap a Promise type in some cases.

Perhaps the biggest pain is the ecosystem. There's no tool to generate libdefs and the ones in flow-typed are often crap or non-existent, so people stub things all the time.

Another sore point is lack of transparency wrt project direction. It's a Facebook-first project and it shows. Things break between patch versions. One really bad side effect is that it's essentially impossible to make semver guarantees for libraries when the type system breaks if the consumer project uses a different version of flow or even simply different configurations.

For us, migrating from earlier versions to flow's Types-First architecture is such a monumental task that we actually decided that migrating our 10+ MLOC monorepo to Typescript would be cheaper.

Also despite the claim of higher strictness, it can still lie to your face. One time, I had to submit a fix to flow-typed of all projects because the maintainer did a refactor to fix a type checker nag and accidentally broke the runtime in doing so.

Ask HN: Do you use Flow (JS type checker) in 2023?

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