It's not external to TS. You write your types by passing object literals to the functions that generate the validators; TypeScript then infers shockingly precise types, which can be extracted using TypeScript's type manipulation utilities.
It does support generics [0].
[0] https://github.com/gcanti/io-ts/blob/master/index.md#generic...
In Show HN: Ezno, a type checker for JavaScript and optimiser for Reactgcanti
io-ts
TypeScript2017–2022
gcanti / io-ts
io-ts
Runtime type system for IO decoding/encoding
40Distinct people
61Mentions
0Explicit recommenders
6.8KGitHub stars
What to know before exploring
What it is
Runtime type system for IO decoding/encoding
GitHub
Where it is used
No documented product use was verified in the last source check.
What people are saying
No recommendation phrase was found in the indexed comments.
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Also in these discussions
- runtypes/runtypes · linked in 7 of the same discussions
- gajus/flow-runtime · linked in 2 of the same discussions
Mentioned in the same Hacker News threads. That is not proof they are substitutes.
This week on Hacker News
Nov 7–Nov 14 UTC · Direct repository links and separately labeled related project stories. Indexed through 11/14/2026, 5:04:57 PM UTC.
No direct repository links were found in the indexed HN discussions over the past seven days. Older discussions are available below.
Mentions over time
HN mentions by year61 dated mentions
Observed mentions: 2017-04-12 02:46:33 UTC to 2022-09-23 18:51:02 UTC.
Counts come from the repository’s dated source evidence. Index cutoff: 2026-11-02 08:16:59 UTC.
View mention counts and date ranges
| Period | Start (UTC) | End (exclusive, UTC) | Mentions | Submissions | Comments |
|---|---|---|---|---|---|
| 2017 | 2017-01-01 00:00:00 UTC | 2018-01-01 00:00:00 UTC | 3 | 0 | 3 |
| 2018 | 2018-01-01 00:00:00 UTC | 2019-01-01 00:00:00 UTC | 5 | 0 | 5 |
| 2019 | 2019-01-01 00:00:00 UTC | 2020-01-01 00:00:00 UTC | 13 | 1 | 12 |
| 2020 | 2020-01-01 00:00:00 UTC | 2021-01-01 00:00:00 UTC | 23 | 0 | 23 |
| 2021 | 2021-01-01 00:00:00 UTC | 2022-01-01 00:00:00 UTC | 14 | 1 | 13 |
| 2022 | 2022-01-01 00:00:00 UTC | 2023-01-01 00:00:00 UTC | 3 | 0 | 3 |
From the project README
An excerpt from the repository, fetched Nov 13, 2026.
Read the project’s introduction
Installation
Usage
Stable features
Experimental features (version 2.2+)
Installation
To install the stable version
npm i io-ts fp-ts
Note. fp-ts is a peer dependency for io-ts
Usage
Stable features
Documentation of the main stable features (index.ts module)
Experimental modules (version 2.2+)
Experimental modules (*) are published in order to get early feedback from the community, see these tracking issues for further discussions and enhancements.
The experimental modules are independent and backward-incompatible with stable ones.
Decoder.ts module
Encoder.ts module
Codec.ts module
Eq.ts module
Schema.ts module (advanced feature)
(*) A feature tagged as Experimental is in a high state of flux, you're at risk of it changing without notice.
Read the full README on GitHubWhat Hacker News says
61 sourcesYou can use io-ts [0] to define your types, and it'll generate functions to typecheck for you. Syntactically it's a bit gnarly and the documentation isn't great; a first-party solution would definitely be nicer. But it works, and it's amazing that it works.
[0] https://github.com/gcanti/io-ts
In Show HN: Ezno, a type checker for JavaScript and optimiser for ReactThat works wonderful as long as you have a flexible type system like Typescript. For me io-ts[0] has been a godsend when I want runtime type-checking on io boundaries (and you may have a lot of them in a front-end application).
[0]: https://github.com/gcanti/io-ts/blob/master/index.md#typescr...
In We don't need runtime type checksConsider using https://github.com/gcanti/io-ts to help with that...
In Things Go needs more than genericsThat's nowhere near "falling apart". That's just the simple fact there's no silver bullet.
Someone arguing a little defensive programming is equivalently strong to a type system is clearly unaware just how much work a good type system does for you. I of course agree with you: when you're fetching data that you don't know the type of, recklessly casting it to some type is going to cause issues. This is true in every language that has ever existed. It's also why tools like IO-TS[0] exist, and of course you can enforce this with JSON Schema techniques or custom validators or a million other options.
Edit: in case the ultimate point of this comment is not clear, by using a type system and some type validator on your fetches, you are able to reduce the need for defensive programming exclusively to your fetch points. Clearly, defensive programm
Read the rest
ing at the fetch points was already needed, so this is why I do not agree with the claim TypeScript's value add disappears from remote fetches.
[0]: https://github.com/gcanti/io-ts
Check out https://github.com/gcanti/io-ts/blob/master/index.md instead. I find it more composable and you can define a codec and get a native type from it so you are only defining things once.
In Parse, Don't Validate (2019)Don’t forget https://github.com/gcanti/io-ts
In Parse, Don't Validate (2019)Use io-ts: https://github.com/gcanti/io-ts
You define a decoder schema, and then the resulting TS type gets automatically derived for you. You can then run data through the decoder, it will err if there's a mismatch, or return a value of the inferred type otherwise.
In Parsix: Parse Don't ValidateSimilar thing for TypeScript: https://github.com/gcanti/io-ts
In Parsix: Parse Don't Validatehttps://github.com/gcanti/fp-ts
https://github.com/gcanti/io-ts
If you're using TS these are pretty much footgun eliminator.
In What's in the Box?Sure, if you're validating some data there're loads of better ways to do it than a bunch of conditionals. I was purely commenting on the switch(true) pattern vs some "if"s.
That approach looks nice though. On that subject, JS has some nice libraries including io-ts[1] which has a functional approach using Eithers to encapsulate errors/success.
[1]: https://github.com/gcanti/io-ts
In Using the switch(true) pattern in JavaScriptDoing runtime-typechecks really is a blessing, and it would be great if typescript would provide this as a language-feature. There are two more libraries I know which are great for that:
* Zod: https://github.com/colinhacks/zod
* io-ts: https://github.com/gcanti/io-ts (for the more functional-programming-oriented)
In How an Anti-TypeScript “JavaScript developer” like me became a TypeScript fanType guard in the example should use runtime type checkers or codec like io-ts (https://github.com/gcanti/io-ts), otherwise it will just become a type-assertion with partial correctness.
With codec, one can make sure that the type is correct and defining type and type-guard becomes a one-time task.
In Bad TypeScript HabitsAlso, for using type guards to validate incoming data, check out https://github.com/pelotom/runtypes.
Or if brave enough to dive into the deep end of FP, io-ts is nice. https://github.com/gcanti/io-ts
In Bad TypeScript HabitsI think the idea is that you validate the data once, on an API level. You don't need to validate the same data again after the initial inspection.
In our team we use io-ts to validate all of our endpoints, but simple type guards could achieve the same goal.
https://github.com/gcanti/io-ts
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