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Function returning a key with type never can be assigned to a typed Object #58565
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To clarify since this is a common point of confusion: |
Or a property that throws upon access. It's fine to create a throwing getter. |
Thanks for your fast replies :)
That is exactly my point. Yet, the example creates such a value of the impossible I fail to see, that assigning an impossible type to a possible type can be a correct, type safe assignment. I feel like, I am missing something here? The |
The problem with the example, and why itβs not an error as written, is because |
Itβs a bit weird, but the mathematical explanation is that itβs the same as how the empty set is a subset of all other sets. More prosaically, if you have a function of type |
Long story short: We can close this issue as works as intended, right? Thanks a lot for your detailed answers! :) Nevertheless, I think it would be useful, if assigning an impossible value to something would be a caught error by the compiler. I see no intentional use of writing a program, whose static types cannot reflect its runtime types. From my point of view, programming languages should be useful and help me to prevent errors: In the example Do you think a feature request for that would be valid? If not, I will trust your judgement π P.S.:
I think you missed the implicit cast due to the type annotation in the function header π P.P.S: Just in case someone finds this helpful: Instead of mapping the not JSONable keys to |
You still seem to mistakingly believe that having a type T = { value: never }
const t: T = { get value(): never { throw new Error() } } |
That's not a cast, it's just a normal return type annotation. If The bottom type means you can never access this value without throwing. As long as that invariant is maintained, then any assignment from |
The ideal solution for this use case would be #23689, but I don't know if we'll ever get it |
awesome, I learned something. I agree that it works as intended and #23689 is the feature I am looking for. Thanks a lot for taking the time! |
π Search Terms
never, function, assignable, assign
π Version & Regression Information
β― Playground Link
https://www.typescriptlang.org/play/?ts=5.5.0-dev.20240518#code/C4TwDgpgBAMghgIwPYCcIqgXigbygd1QGsAuKACgEosA+KAOwFcBbBdKAXwG4AoUSWHABeIeMjQZseQilIMIAN3bcePAGaN6AY2ABLJPSgBzCMDGp05AM7pdcADa6hEACYA1B4whkrwFLvojSjJ4EXMJXB4oKDRgRhRDACkAZQB5ADkAOjA4FBtrWwcnVw97L0peDlV7UyhmEAB1YnQQxAsUXnqm2XZsEzM2iXIAIkyx4YqoAHopqGAAC10rKCt5pEZ7FwYkYCg2KAd7JHxXABoD5bAUJEgUUChhmSJhqCW58Gh6RXYAh9DRQboYY8LrNFCZJ5ULjTWb4XT2exzebXfAHXYoTR6ZgQHhAA
π» Code
π Actual behavior
The code compiles without an error.
π Expected behavior
The code should not compile, as we assign
work: never
to a real object.Additional information about the issue
My real use case is, that I want to check, if a type is serializable with JSON. Inspired by the blog mastering-type-safe-json-serialization-in-typescript, I wanted to write a small library like the following
And the real code would look like the following:
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