MapTypes

Problem Link

Problem

Implement MapTypes<T, R> which will transform types in object T to different types defined by type R which has the following structure:

type StringToNumber = {
  mapFrom: string  // value of type string
  mapTo: number    // will be transformed to type number
}

type SetMixed = MapTypes<{isFun: boolean, year: string}, {mapFrom: boolean; mapTo: string} | {mapFrom: string; mapTo: boolean}>
// {isFun: string, year: boolean}

Solution

Approach: Mapped Type with Union Lookup

For each property, find the matching mapFrom in R and use the corresponding mapTo.

type MapTypes<T, R extends { mapFrom: unknown; mapTo: unknown }> = {
  [K in keyof T]: T[K] extends R['mapFrom']
    ? R extends { mapFrom: T[K]; mapTo: infer To }
      ? To
      : never
    : T[K]
}

How it works:

1. For each key K, check if T[K] is a valid mapFrom value.
2. If so, filter R to the specific variant where mapFrom matches T[K], and extract mapTo.
3. If no match, keep the original type T[K].
4. When R is a union, R extends { mapFrom: T[K] } distributes and collects all matching mapTo types.

Key Takeaways

• Filtering a union with R extends { mapFrom: T[K] } distributes over the union and keeps only matching variants.
• This is the type-level equivalent of find on an array of mapping rules.
• When multiple rules match, the result is the union of all mapTo types.