Typescript: a object only allow one Specific key

Question

I want to define a object only have one specific key，and the key depend on a type, for example：

type keys = 'a' | 'b' | 'c'

// thats ok
obj = {
 a: 'string'
}

// no
obj = {
 a: 'string',
 b: 'string'
}

// no
obj = {
 a: 'string',
 d: 'string'
}

// no
obj = {
 d: 'string'
}

I can only think of one way like：

type MyObject = {
 [key in keys]?: string
}

but its just half-measures, so is there a good or perfect way?

Answer 1

It is possible to do this by using some types to create a discriminated union of values.

type CreateObjHelper<Keys extends string> = {
    [K in Keys]: {
        [K2 in Keys]?: K2 extends K ? string : never
    }
}
type CreateObjOneKey<Keys extends string> = CreateObjHelper<Keys>[keyof CreateObjHelper<Keys>]

type MyKeys = 'a' | 'b' | 'c'

type MyObjOneKey = CreateObjOneKey<MyKeys>

const obj: MyObjOneKey = {
    a: ''
}
//...

It will work on the test cases you've given here.

See it in action on TS Playground