How tipisation closures

Question

const createSuffixName = <N extends string>(name: N) => {
  return <S = undefined>(suffix?: S): S extends string ? `${N}/${S}` : N => {
    return suffix ? `${name}/${suffix}` : name;
  };
};

const test = createSuffixName<'test'>('test');
const testName1 = test();
// testName1 type = "test"
const testName2 = test<'suffix'>('suffix');
// testName2 type = "test/suffix"

there is an error:

return suffix ? `${name}/${suffix}` : name;

--

TS2322: Type 'N | `${N}/${string}`' is not assignable to type 'S extends string ? `${N}/${S}` : N'.
  Type 'N' is not assignable to type 'S extends string ? `${N}/${S}` : N'.
    Type 'string' is not assignable to type 'S extends string ? `${N}/${S}` : N'.

I don't know how to fix it so that the behavior remains the same.

Does anyone have any ideas how to fix this? Thanks.

The root of the problem is the same as: [How to fix TS2322: "could be instantiated with a different subtype of constraint 'object'"?](https://stackoverflow.com/q/56505560/251311) — zerkms, Oct 24 '22 at 22:45
And in short - that expression (effectively of a string type) is not assignable to the `S extends string ? \`${N}/${S}\` : N` because `N` is not necessary a string (but something that extends it). — zerkms, Oct 24 '22 at 22:46

How tipisation closures

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