[Feature] Utility type for T[keyof T] or new keyword valueof #37642

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1. Ryan Cavanaugh (RyanCavanaugh)

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   SuggestionAn idea for TypeScriptAn idea for TypeScript

   on Mar 27, 2020
2. devinrhode2 commented on Dec 28, 2020

   

   Devin Rhode (devinrhode2)

   on Dec 28, 2020

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   I wonder why valueof wasn't introduced at the same time as keyof?
3. JoshuaKGoldberg commented on Mar 4, 2022

   

   Josh Ghoulberg 👻 (JoshuaKGoldberg)

   on Mar 4, 2022

   Contributor

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   To add to the reasons & voices in favor: a valueof keyword or similar would be quite useful with getting enum-like behavior with const objects.

   Today, if you'd like to grab the value out of a const object, you have to use something akin to (typeof MyObject)[keyof typeof MyObject]:

   const StatusCodes = {
       InternalServerError: 500,
       NotFound: 404,
       Ok: 200,
       // ...
   } as const;
   
   // Type: 200 | 400 | 500
   type StatusCodeValue = (typeof StatusCodes)[keyof typeof StatusCodes];
   
   let statusCodeValue: StatusCodeValue;
   
   statusCodeValue = 200; // Ok
   
   statusCodeValue = -1;
   // Error: Type '-1' is not assignable to type 'StatusCodeValue'.

   A valueof syntax would be much cleaner:

   // Type: 200 | 400 | 500
   type StatusCodeValue = valueof typeof StatusCodes;

   👍React with 👍2Reacted by Dmytro Lyfarenko and Piotr Kowalski
4. WillsterJohnson commented on Jul 27, 2022

   

   Willster Johnson (WillsterJohnson)

   on Jul 27, 2022

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   valueof as a keyword makes so much sense, I feel like having a keyword in line with Object.values is a logical progression from keyof being in line with Object.keys.

   In fact, why don't we have some more keywords for some of the most used ObjectConstructor methods?

   // these exist
   
   Object.keys(obj) -> keyof typeof obj
   
   // this one uses the utility type `Readonly<T>` rather than `readonly T`, close enough
   Object.freeze(obj) -> readonly typeof obj
   
   // this feature request
   
   Object.values(obj) -> valueof typeof obj
   // provides "v1" | "v2"
   type ValueOf<T> = T[keyof T];
   
   // possibly more?
   
   Object.entries(obj) -> entryof typeof obj
   // provides ["k1" | "k2", "v1" | "v2"] (not ideal: allows ["k1", "v2"] which is not a valid entry)
   type EntryOf<T> = [keyof T, valueof T];
   // provides ["k1", "v1"] | ["k2", "v2"] (preferred: associates 1:1 key:value) (hard to read as a util - doesn't matter as a keyword)
   type EntryOf<T> = valueof { [K in keyof T]: [K, T[K]] };
   // note that these won't be too different in length of generated type for reasonably sized objects

   (I should note that I'm not asking for the definitions on ObjectConstructor to be changed to reflect the listed types, I'm simply listing the similar keywords. I believe definition changes have already been discussed elsewhere, plus they're off-topic here so let's leave it at that.)

   In all honesty, I went through every method under interface ObjectConstructor { ... } and entries was the only other one that seems reasonable.
   (Drifting off-topic slightly) Trying to think of other keywords, I can see that the useful ones I've been able to think of have their respective issues. (if you're interested, I found these: writable, public-ify, nameof)

   It's common to cast Object.keys(obj) as keyof typeof obj, or do the same for values and entries, having a way to do so more easily for the latter two would be helpful for everyone who's casting the outputs, and it wouldn't involve changing the type definitions.