Theo's Typesafe Cult•12mo ago

Problem with TS conditional on union

I'm currently playing around with typescript conditionals, and doing conditionals on string literal unions isn't quite working for me

type EventsType = 'ready' | 'data' | 'reset' | 'end'

type Discriminate<T extends EventsType> = T extends 'reset' | 'end'
  ? () => void
  : dosomth<T>

type dosomth<T extends 'data' | 'ready'> = (data: string, type: T) => void

type EventsType = 'ready' | 'data' | 'reset' | 'end'

type Discriminate<T extends EventsType> = T extends 'reset' | 'end'
  ? () => void
  : dosomth<T>

type dosomth<T extends 'data' | 'ready'> = (data: string, type: T) => void

This is the code that i have, but for some reason, TS is complaining that

typescript: Type 'T' does not satisfy the constraint '"data" | "ready"'.
  Type 'EventsType' is not assignable to type '"data" | "ready"'.
    Type '"reset"' is not assignable to type '"data" | "ready"'. [2344]

typescript: Type 'T' does not satisfy the constraint '"data" | "ready"'.
  Type 'EventsType' is not assignable to type '"data" | "ready"'.
    Type '"reset"' is not assignable to type '"data" | "ready"'. [2344]

in the dosmth<T> line https://www.typescriptlang.org/play?#code/C4TwDgpgBAogbhAdsAzgFXNAvFA5AJwgEMATEXKAHzxKOCIuoIhQmEbyRNwCgfRIUACIBLFAGN8IgLYjEdCAB40UCAA9gXFLATJ0mAHxQcK9ZsQltzVuyqcLvKFAD8UABQBKY0bgB7ESQ8TgBcUCS+KL7SwAAWygZ8AtDhkdFxphpaNHQMdsyk5EY4brT0oSjAUogA5gA0UEmhaF5YPv6BPEA here is the typescript playground. What am i doing wrong? Why isn't it correctly taking the 'reset' string literal out of T?

TS Playground - An online editor for exploring TypeScript and JavaS...

The Playground lets you write TypeScript or JavaScript online in a safe and sharable way.

Solution:

for some reason typescript does not understand that T can only be data or ready just check for it again: https://www.typescriptlang.org/play?#code/C4TwDgpgBAogbhAdsAzgFXNAvFA5AJwgEMATEXKAHzxKOCIuoIhQmEbyRNwCgfRIUACIBLFAGN8IgLYjEdCAB40UCAA9gXFLATJ0mAHxQcK9ZsQltzVuyqcLvKFAD8UABQBKY0bgB7ESQ8TgBcUKYaWjR0DHbMpOQuUCS+KL7SwAAWykahiBAI+HwC0Mmp6Vnh5pZR9BxxZLhGOG609KEowFKIAOYANFDFoWheWD7+gTxAA...

TS Playground - An online editor for exploring TypeScript and JavaS...

The Playground lets you write TypeScript or JavaScript online in a safe and sharable way.

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