typescript - Typings for an object where keys and values are the same

Question

Is there a way in typescript to express the typings of a function that takes in a list of strings, and creates an object where each key's value is the key itself:

const createActionTypes = (types: string[]) => {
  return types.reduce((typesMap, type) => {
    typesMap[type] = type
    return typesMap
  }, {})
}

// Input
createActionTypes(['a', 'b'])

// Output
{ a: 'a', b: 'b' }

Here's what I've tried, without luck:

const createActionTypes = <T extends readonly string[], U extends T[number]>(
  types: T
): { [key in U]: U } => {
  return types.reduce((typesMap, type) => {
    typesMap[type] = type
    return typesMap
  }, {})
}

const x = createActionTypes(['a', 'b'] as const)
// Typings for x get resolved to: 
{
  a: "a" | "b";
  b: "a" | "b";
}

question from:https://stackoverflow.com/questions/66065173/typings-for-an-object-where-keys-and-values-are-the-same

Answer 1

You are really close. You need to specify that the value is same type as key and not the whole set "a"|"b" which is U.

TS Playground Link

const createActionTypes = <T extends readonly string[], U extends T[number]>(
  types: T
): { [key in U]: key } => {
  return types.reduce((typesMap, type) => {
    typesMap[type] = type
    return typesMap
  }, {} as any)
}

const x = createActionTypes(['a', 'b'] as const)

_{Also, just for clarity, I would avoid using reserved words like type as a variable or function parameter. It required me to look twice to ensure type is not type, but a value.}