Typescript: Using conditional types and optional variables to map one string to another

Question

I am trying to create a generic function that can essentially map one string to another at compile time. If any string is inputted, then " data" should be returned, if undefined, then null should be returned. The problem is based on making `input file optional.

I have it almost working w/with different functions, but I have test cases that fail for both that I have marked with error below. I also have a TS playground setup.

type OutputType<T> = T extends string ? "data" : null;

function map<T extends string | undefined, V extends OutputType<T>>(
  inputFile: T
): V {
  return (inputFile ? "data" : null) as V;
}

function mapOptional<
  T extends string | undefined = undefined,
  V extends OutputType<T> = OutputType<T>
>(inputFile?: T): V {
  return (inputFile ? "data" : null) as V;
}

const map1 = map("file2");
assertTrue<TypeEqual<"data", typeof map1>>();
const map2 = map("x" as string | undefined);
assertTrue<TypeEqual<"data" | null, typeof map2>>();
const map3 = map(undefined);
assertTrue<TypeEqual<null, typeof map3>>();
const map4 = map(); // error
assertTrue<TypeEqual<null, typeof map4>>(); // error

const mapOptional1 = mapOptional("file2");
assertTrue<TypeEqual<"data", typeof mapOptional1>>();
const mapOptional2 = mapOptional("x" as string | undefined);
assertTrue<TypeEqual<"data" | null, typeof mapOptional2>>(); // error
const mapOptional3 = mapOptional(undefined);
assertTrue<TypeEqual<null, typeof mapOptional3>>();
const mapOptional4 = mapOptional();
assertTrue<TypeEqual<null, typeof mapOptional4>>();

Please mark where you have expected errors

captain-yossarian from Ukraine
– captain-yossarian from Ukraine

2021-05-23 17:48:50 +00:00
Commented May 23, 2021 at 17:48 — captain-yossarian from Ukraine
– captain-yossarian from Ukraine, Commented May 23, 2021 at 17:48
I already marked them with error

Sharpiro
– Sharpiro

2021-05-23 17:55:01 +00:00
Commented May 23, 2021 at 17:55 — Sharpiro
– Sharpiro, Commented May 23, 2021 at 17:55

Joonas · Accepted Answer · 2021-05-23 18:32:27Z

1

It's a tricky one, because the optionality of the parameter is not explicitly part of its generic type.

I managed to work around that by just using multiple function declarations:

function mapOptional(): null;
function mapOptional(inputFile: undefined): null;
function mapOptional<T extends string>(inputFile: T): 'data';
function mapOptional<T extends string | undefined>(inputFile: T): 'data' | null;
function mapOptional<
  T extends string | undefined
>(inputFile?: T): 'data' | null {
  return (inputFile ? "data" : null) as any;
}

answered May 23, 2021 at 18:32

Joonas

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