Let’s say I have a Map with keys A and B that each store a boolean value, and keys X and Y that each store their own array of string. The simplest way to type this would be to write:

type CustomType = Map<string, boolean | string[]>

However, if I write the following code wherein I’ll know the value type, TypeScript will be uncertain of the values type:

for (const [key, value] of MyMap) {
  switch(key) {
    case 'A' || 'B':
      // value will only be boolean but typescript thinks the value
      // could still be a string[],
      break
    case 'X' || 'C':
      // and vice versa.
      break
  }
}

Using a union as the following tells TypeScript that the Map as a whole will only be one or the other:

type CustomType = Map<'A' | 'B', boolean>
                | Map<'X' | 'Y', string[]>

Switching to intersection type seems to ignore the second half.

type CustomType = Map<'A' | 'B', boolean>
                & Map<'X' | 'Y', string[]>

Does TypeScript have a way to properly type this that doesn’t require adding unnecessary type checks or littering my code with @ts-ignore comments?