TypeScript: Return type has all keys specified as parameter

Question

I would like to type a function in a way, that the return type will be an object, that contains exactly the keys specified in an array as a parameter. Also these keys should all be of a specific enum type. I've already tried a couple of things, but can't get it working. Maybe someone knows if it's possible at all and how?

enum Status {
  TIME = 'time',
  LOCATION = 'location'
}

function getStatus(statusToGet: Status[]) {
  // This function will always return an object containing all passed in
  // status as a key and boolean as a value, e.g.
  return statusToGet.reduce((prev, stat) => {
    prev[stat] = true; // or false
    return prev;
  }, {});
}

getStatus([]) // -> {}
getStatus([Status.TIME]) // -> { time: true }
getStatus([Status.TIME, Status.LOCATION]) // -> { time: true, location: false }

Is it possible to type the getStatus function correctly, so that TS knows which keys are in the return type?

Answer 1

You can use mapped types to map an array of string literal types to an object :

enum Status {
    TIME = 'time',
    LOCATION = 'location'
}

function getStatus<T extends Status>(statusToGet: T[]): { [P in T]: boolean } {
    return null as any;
}

getStatus([]) // -> {}
getStatus([Status.TIME]) // -> { time: true }
getStatus([Status.TIME, Status.LOCATION]) // -> { time: true, location: false }

Note The above code returns { [s:string] :any} for an empty array if strict null checks are not enabled. This happens because T will be inferred to any . To get around this we can use a conditional type to test if T is any and return {} explicitly (10x to @jcalz for the any test from this answer ). This does mean any array of any will produce {} but since we expect an array of enum values not an array of any this migh not be too much of a problem, might actually be a feature.

function getStatus<T extends Status>(statusToGet: T[]): true extends false & T?  {} : { [P in T]: boolean } {
     return null as any;
}