The question was about creating indexed type assignable to string. For the full question please visit Advanced TypeScript Exercises - Question 8

The Answer

const concatToField =
  <T extends Record<K, string>
  , K extends keyof T >(obj: T, key: K, payload: string): T => {
    const prop = obj[key];
    return { ...obj, [key]: prop.concat(payload) }; // works 👍
}

The key to the problem was obj[key] which is a type of T[K] so the whole problem comes to - how to ensure T[K] is always string. Trying to narrow the K type only for string values in T will not work, as T can have no string fields at all, so we can end by never(the bottom type).

The simplest solution is restricting T to be extending Record<K, string>, what does it mean - we say that our T needs to have key K being a string. Now if we put key which will be having different value than string there will be compilation error.

Full solution available in the playground

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