Fast and precise type checking for JavaScript
blog.acolyer.org
blog.acolyer.org
Microsoft did a really good job putting resources behind TypeScript, making sure tooling and IDE integrations are good, and generally getting a ton of momentum going. TypeScript is fast-moving, with bugs fixed and features added at a high rate, and the quality of discussion on the issue tracker is high. Flow, in comparison, was not evolving as fast.
Flow touts global inference, which I think means better types when you don't necessarily have annotations on module boundaries, but I mostly care about what's possible in a greenfield or well-annotated codebase.
Flow deserves a ton of credit, and Microsoft probably studied it in detail, but I think the advantages are overblown at this point. Take the soundness thing. TypeScript added null-strictness a while ago, and recently they improved function type polymorphism. The reality is that both type systems have limits and you sometimes need to type something imperfectly or use an escape valve ("any"). In most cases, however, TypeScript gives you more sophisticated tools to construct the types you want, so you actually get better types. Maybe TypeScript lacks a theoretical underpinning for soundness, but there's no reason it can't continue to get "more and more sound," with it harder and harder to find good examples of unsoundness.
Can't speak for OP's situation though.
Besides, I feel like if I really want a strong type system https://reasonml.github.io/ might be the better choice over flow.
When working with another person API then typescript does become a godsend in helping understand what the bloody hell does this callback parameter are required.
Plot twist: While you are cursing at the person who wrote that API, you slowly realize that it was you all along.
At my current job we started a clean slate codebase (react/redux/webpack) beginning of 2016 and had everything typechecked by Flow from the start.
We found Flow to be extremely unreliable and cumbersome to use with not much apparent commitment behind it. As soon as Typescript hit 2.0 we converted the entire codebase (fairly easy to do since the syntax and semantics are very similar') and have been super happy with that decision.
I wholly agree on you remark about soundness vs. tooling. Flow tooling sucked, error messages were cryptic and unhelpful and we ran into obvious mistakes all the time that Flow did NOT catch, without being able to figure out why that was. Editor support was lacking and we just never trusted the typechecker.
Bolting on types to Javascript has tradeoffs and Microsoft did an amazing job of making the right tradeoffs.
There is noticeable effort and momentum behind the project that inspires a lot of confidence in the languages future. It has completely changed the way I write Javascript and I can't imagine going back to an untyped world.
That change was not in rock-solid confidence that TS catches all my errors (although it does a remarkably good job at that), but the sheer productivity improvement types enable (autocompletion, project-wide renaming, all that stuff users of Java or C# IDEs have available to them).
It's very difficult get the team's attention to a bug or a question. As a result, they fail to see the common pain points of actual users.
Typing higher order components is one common pain point that still doesn't have a clear solution. So, unless you're avoiding HOCs altogether, you can't really benefit from type annotations in your react codebase (you'll have too many implicit `any`s).
You'll find more of these common use cases that flow doesn't optimise for. They're all over the issues section. Unfortunately, issues on Github generally don't get much attention from the team either, so I'm guessing users sometimes give up on reporting them. This has happened to me quite a few times. I didn't report some bugs because putting together a reduced test case takes time, and that time would be wasted if your issue is not gonna be paid attention to.
What makes matters worse, is that there is no public roadmap. You don't know what the team's priorities are. You don't know when the bugs that affect your work are gonna get fixed, or some feature you need is gonna get implemented. You don't even know if it's on the radar.
Of course, the Flow team is under no obligation to do any of that. They have no obligation to fix bugs for us or publish a roadmap. I'm already grateful that they've given us access to their work and without charging money. However, they do have the responsibility to communicate what their priorities are. If they're positioning Flow as an alternative to TypeScript, which is a well-supported, community-oriented project, then they should state clearly that Flow simply isn't. Call it a research project, or a project focused on a single company's use cases. Don't ask people to bet their own projects on it, when they clearly shouldn't. It'll be a big loss of productivity for them down the line, and your messaging is partly responsible for that.
We migrated to Typescript. Error messages are more understandable and unlike flow include line numbers and the column. Add huge number of typings, and ts definitely wins.
There are many gripes we've had with typescript too, but at least we didn't spend hours trying to understand error messages.
It seems like a useless academic term that doesn't work so well in the real world, maybe?
Here's an example of unsoundness in TypeScript:
https://www.typescriptlang.org/play/index.html#src=function%...
TypeScript incorrectly (but conveniently) says that Array<string> can be assigned to Array<string | number>, and you can exploit that to create an "s" variable that TypeScript thinks is a string but is actually a number.
You can try the same code in Flow and it gives a type error:
https://flow.org/try/#0GYVwdgxgLglg9mABAWwKYGd0FUAOAVAC1QEEA...
Neither Flow nor TypeScript are correct in this instance. Neither keep track of the actual array element's value, just the general type of the array, which means they actually don't know for sure and do their best guess. So in the Flow example, it complains that the number and string types are incompatible even though it doesn't know that this specific case is incompatible, just the general case. In the TypeScript example, it should keep track of the type of the argument value supplied during the function invocation, not the type for the argument declaration.
In this case I would argue that even though TypeScript is incorrect, it's preferable to Flow because Flow doesn't infer that the types are incompatible from actual usage but theoretical.
https://flow.org/try/#0GYVwdgxgLglg9mABAWwKYGd0FUAOAVAC1QEEA...
Both Flow and TypeScript have good type inference (with Flow's generally being better, I think) and do pretty well with all type annotations removed, but that wasn't shown in my example because I explicitly annotated all types.
Note that if you do want/need to give an explicit type annotation for this sort of thing, Flow provides `$ReadOnlyArray`, where `Array<string>` is assignable to `$ReadOnlyArray<string | number>`:
https://flow.org/try/#0GYVwdgxgLglg9mABAWwKYGd0FUAOAVAC1QEEA...
It sounds like you're arguing that TypeScript is wrong because it's overly-permissive, and Flow is wrong because it's overly-strict, which makes sense. That's probably why people prefer to use the word "sound" to describe Flow rather than "correct". Every sound type system has cases where you can write perfectly correct code that would be rejected by the type system (which is provable because of the halting problem). Opting into a type system always means that you limit the type of code you can write in exchange for better automatic verification.
This example is quite interesting because javascript itself does not provide a way to check the type of an array, unlike a primitive. This is likely due to the fact that an empty array doesn't really have a type for it's items yet.
Here is an example where typescript can infer the type using the `typeof` runtime check but provide compile time checking.
https://www.typescriptlang.org/play/index.html#src=function%...
That's because the author told Flow explicitly to expect an array of strings and numbers.
class Dog {
}
class Greyhound extends Dog {
doGreyhoundThing(): void {
console.log("I am a greyhound!");
}
}
class Poodle extends Dog {
doPoodleThing(): void {
console.log("I am a poodle!");
}
}
function f(g:(Dog) => void) : void {
let hound: Greyhound = new Greyhound();
g(hound);
}
function h(p: Poodle): void {
p.doPoodleThing();
}
f(h);
Flow has already long-since demonstrated that it's perfectly possible to introduce useful type annotations to JS code without changing any runtime behavior whatsoever, and Flow and TypeScript have mostly converged on similar syntax and semantics when it comes to the type annotations. Given all this, I'd have thought that the standardization process would be pretty far along by now. Maybe someone more familiar with these matters can offer some insight on the seeming lack of progress?
[1] https://en.wikipedia.org/wiki/ECMAScript#4th_Edition_.28aban... [2] https://www.ecma-international.org/activities/Languages/Lang...
And once type annotations are part of the language proper, the possibility opens up for browsers to use type annotations to provide runtime safeguards, optimizations, and reflections, all of which would be invaluable since static checking alone is often not enough to provide correctness guarantees in all cases in JavaScript. When all is said and done, JavaScript is a dynamic language, and the ability to reuse the same type annotations for both static and dynamic checks is the piece that I'm sorely missing from both the Flow and TypeScript ecosystems so far.
The immediate benefit to adding Typescript/Flow-like annotations to the ES spec would be that you could run Typescript files unchanged (without transpilation) in the browser, even if the Browser didn't do any type checks for you. You could still use TS or Flow to do the type checks in a separate process, but you could potentially drop the type-stripping steps in TS or Babel. The possibility exists that eventually the browser could also start to enforce basic type checks, but the immediate benefit of slightly faster build processes with no type-stripping step shouldn't be overlooked.
This would be best case scenario so we could get rid of the tedious work that one has to put in to make an older library have type checking.
That inference is also made available more directly in dts-gen [1], which is a tool built around Typescript inferencing that produces decent first pass type definition files for a library.
dts-gen may be something to try the next time you need a definition file. (Personally, I still prefer to start a new definition file by hand, but it's good to have an automated option, even if only to double-check your work.)