Support `satisfies` operator on functions

[dummdidumm]

Suggestion

🔍 Search Terms

satisfies function

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Support the satisfies operator to appear after a function

function foo () { } satisfies Bar

This was requested/mentioned in #47920 several times but never followed up on

📃 Motivating Example

satisfies is super handy to use, and you can already use it for arrow functions. Not having support for it on functions feels unintuitive and forces a certain coding style where people have to use arrow functions when they rather want to use functions.

💻 Use Cases

Typing a function while preserving its return type.

[bradennapier]

It is not the prettiest way to define a function but it does the job and the result is awesome - retains the actual types and therefore infers the generic of SpotServicesController whereas if i just did const binance: SpotServicesController i would need to do const binance: SpotServicesController<BinanceTickerResult>

Not to mention the IDE tips become much less useless without jumping around to diff files to track down what the type is:

[gfx]

This feature is absolutely useful for everyone, but there's already a feature request with implements keyword for the same purpose in 2019:

• Describe function signature using an interface  #34319

[xamir82]

Weird that this didn't make it to 4.9 :/
Ryan Cavanaugh (@RyanCavanaugh) Sorry to tag you but just wanted to ask whether there's any chance this will be implemented in the near future? The satisfies feature just feels incomplete without it.

[aradalvand]

This should've honestly been implemented already; such a bummer.

[PooSham]

FUJI Goro (@gfx) the difference is that implements would require you to define the type of the parameters. But anyone goes, the current situation is quite annoying.

[ekaradon]

I was looking for this suggestion. Upvoted it immediately. The first time I had to write some pages with Next, I was just thinking that I would replace lambda by function (for all the benefits that function has over lambda) and thought that satisfies would just work better by not erasing the return type but keeping it type safe nonetheless. Really surprised that an exception for function has been made here.

It's such a recurring use case. It would be awesome to get it. Especially because it would be consistent with just all the others use cases where satisfies is valid. Please make it a reality! 😃

[aradalvand]

Functions are possibly the most common use case for satisfies and they're not supported.
Brilliant.

[hanneswidrig]

What can we do to give this more visibility, I see "awaiting more feedback" is one of the tags on this issue?