Myujth

Now that typescript 3.1 introduced mapped tuple types, I was hoping this code sample would work:

export interface SettingKey 
 General_Language: 'en' 

export function fetchSetting<K extends (keyof SettingKey)>
 (...keys: K): Promise<SettingKey[K]> 
 return null as any;

fetchSetting('General_Language', 'Map_InitialLongitude').then(x => 'sl'
)

But it doesn't. The errors are:

ttt.ts:7:83 - error TS2536: Type 'K' cannot be used to index type 'SettingKey'.

7 export function fetchSetting<K extends (keyof SettingKey)>(...keys: K): Promise<SettingKey[K]> {
 ~~~~~~~~~~~~~

ttt.ts:11:12 - error TS2571: Object is of type 'unknown'.

11 return x['General_Language'] === 'de'
 ~

Clearly the second error is a consequence of the first one, so that's not really a concern. The first one is the problematic one.

keys is an array of keyof SettingKey, and so I would hope that SettingKey[K] would be an array of the types of the listed properties (so, concretely in the code sample I put, it would be ['en' | 'sl', number]. From the pull request introducing the typescript feature:

If T is an array type S we map to an array type R, where R is an instantiation of X with S substituted for T[P].

But that holds I guess for mapped types only, and here I have a lookup type, that would be the reason why it doesn't work I guess?

I think what I want to express is clear; can this be made type-safe in typescript?

To have a mapped tuple you need a mapped type, that will map the original tuple (in the type parameter K) to the new tuple type

export interface SettingKey 
 General_Language: 'en' 

type SettingKeyProp<P extends keyof SettingKey> = SettingKey[P]
type SettingKeyArray<K extends [n: number]: keyof SettingKey > = 
 [P in keyof K]: K[P] extends keyof SettingKey ? SettingKey[K[P]]: never 
 
export function fetchSetting<K extends (keyof SettingKey)>
 (...keys: K): Promise<SettingKeyArray<K>> 
 return null as any;

fetchSetting('General_Language', 'Map_InitialLongitude').then(x => 'sl'
 return x[0] === 'de' /// since you want a tuple, you should index by number not name
)

If you want to index by name that is also possible, but the mapped type should map over the values in the array not the keys in the array:

type SettingKeyArray<K extends [n: number]: keyof SettingKey > = 
 [P in K[number]]: SettingKey[P] 
 
export function fetchSetting<K extends (keyof SettingKey)>
 (...keys: K): Promise<SettingKeyArray<K>> 
 return null as any;

fetchSetting('General_Language', 'Map_InitialLongitude').then(x => 
 // you can access by name
 return x.General_Language === 'de' 
)