type assertion should support additional return type

[zen0wu]

🔍 Search Terms

assert types, type narrowing

✅ 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 isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

asserts return type of a function/method should also be able to return additional types as part of it.

• Maybe a limited form can be returning the type that's being asserted: asserts x is T also returns x (can also be written as x & asserts x is T)
• A more general form would be allowing assert functions to return arbitrary types with assertions: fn(x: string | number): string asserts x is string
• Or maybe we could even support assertions on multiple arguments?

📃 Motivating Example

Use case:
I want the ability to narrow down this in a class so that I can mimic some level of "extension methods".

Here's a playground link: https://www.typescriptlang.org/play/?#code/MYGwhgzhAECyD2ATApiAPPArgF2gFQD5oBvAKGgugDswAnW+Adz3gGVtaBLKgcwAoAlAC5okCMlrYY2ABacY8uElRoIHbjyJlKO6LWTZMtKtA6Zk5SgF9Slimq69YBmUgCMfWfJEIU6BxoEAiQ2NqTA8FRq0AAe0AC81MiMSn5oVJgAtgBGEtAAPtABvASCpAD05fhyMBCumCCI0ABmYJwgAHSkMR3FPM6y7mUVVQCS0IxgVLjY8NC5otkgyKZziHNeEF09NPRMLOyO-AK96k4uQwKkQA

💻 Use Cases

1. What do you want to use this for?

See use-case.
Another use-case is something I came up before (Asserts in constructor) section: #38442

2. What shortcomings exist with current approaches?

Cannot be done.

3. What workarounds are you using in the meantime?

For the motivating example, I have to create extension objects which adds extra runtime cost.

[MartinJohns]

// I want to be able to do this.
x.narrowToString().stringMethod1()

This is clearly out of scope for TypeScript. The narrowToString() method returns a boolean, and that type does not have a stringMethod1(). TypeScript would need to rewrite the code, which is out of scope.

Other than that it sounds like a duplicate of #40562.