Typescript extract type of all unique leaf values of an object

Issue

I have a constant which is a tree holding unique keys as its leaves, e.g.:

const Tree = {
  Branch1: {
    Leaf1: '1'
    SubBranch1: {
      SubLeaf: '11'
    },
    ...
  },
  Branch2: {
    Leaf2: '2'
    SubBranch2: {
      SubLeaf: '21'
    }
  },
  ...
}

I would like to create a generic type Leaves<typeof Tree> that would be a list of all of the leaf values, i.e. '1' | '11' | '2' | '21' .... The issue is that I cannot find any examples of how I can effectively reduce a tree of objects to their keys or anything of the sort.

Solution

You may use

const Tree = {
  Branch1: {
    Leaf1: '1',
    SubBranch1: {
      SubLeaf: '11'
    }
  },
  Branch2: {
    Leaf2: '2',
    SubBranch2: {
      SubLeaf: '21'
    }
  }
} as const;

type Leaves<T, L0 = T[keyof T]> = L0 extends Record<string, any> ? Leaves<L0> : L0;

let l: Leaves<typeof Tree>
// ^?let l: "1" | "11" | "2" | "21"

playground

The only thing worth explaining is the fact that the type parameter L0 may be a conditional type (if there are multiple keys) so in that case it works as a distributive conditional type.

Also, using as const (const assertion) when defining the constant ensures its type is the most specific case, otherwise the leaves types would have been just string.

Note: edited to remove unnecessary code. The original playground might still be useful to understand how it works.

Answered By - kikon