Typescript trying to infer with generics for objects

Question

This is what i am trying to accomplish but for an object with generics. Is there a `valueof` similar to `keyof` in TypeScript?.

// Something on this lines. I know this is not supported but i am trying to convey the idea here.

type payload<T> = <K extends keyof T>{prop: K, value: T[K]};

const someObj = {a: string, b: number};

type someObjType = payload<someObj>;

const someObjPayload: someObjType = { prop: 'a', value: 'some string'} // should work.

const someObjPayload: someObjType = { prop: 'a', value: 200 } // should throw an error.

Titian Cernicova-Dragomir · Accepted Answer · 2018-09-26 08:05:08Z

1

Unfortunately typescript doesn't allow partial inference for variables. If you want to ensure that the value is of the same type as the property specified by key you will need o use a helper function to get the apropriate inference behavior:

type payload<T, K extends keyof T> = {prop: K, value: T[K]};

let someObj!: {a: string, b: number};

function getProp<K extends keyof typeof someObj>(o: payload<typeof someObj, K>) {
  return o
}

const someObjPayload = getProp({ prop: 'a', value: 'some string'}) // ok of type payload<{ a: string; b: number; }, "a">

const someObjPayload2= getProp({prop: 'a', value: 100}) // error

answered Sep 26, 2018 at 8:05

Titian Cernicova-Dragomir

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Nigel Nop · Accepted Answer · 2018-09-26 07:53:57Z

1

Since only number and string are allowed in TypeScript, it might be simpler to just write everything two times - leaving your type parameter K out

type stringKeyPayload<T> = {prop: string, value: T}
type numberKeyPayload<T> = {prop: number, value: T}

answered Sep 26, 2018 at 7:53

Nigel Nop

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3 Comments

Nigel Nop Over a year ago

Sorry for the lame answer :D I recently had a similar problem and came up with that solution

Titian Cernicova-Dragomir Over a year ago

The problem with your answer is that value should be a type taht is a property type of T not T (ie T[keyof T]) And the problem OP wants to solve is to ensure the value is of the same type as the property specified by K ..

Nigel Nop Over a year ago

Ah, I see. Thanks for pointing that out. @Titian's answer looks way better

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