In what way can I utilize const assertions to extract literal types from deeply nested objects?

Question

In what way can I utilize const assertions to extract literal types from deeply nested objects?

Let's imagine I have a unique set of individuals:

const group = {
  Alex: ['Ben', 'Chris'],
  Sarah: ['Dylan'],
} as const;

With the help of constant clarifications, I can define the specific types like so:

type Parent = keyof typeof group; // Alex|Sarah
type Child = typeof group[Parent][number]; // Ben|Chris|Dylan

Now consider this different set, with another layer added:

const offspring = {
  Alex: {
    Ben: ['Ella', 'Fiona'],
    Chris: ['Hannah'],
  },
  Sarah: {
    Dylan: ['Isabella'],
  },
} as const;

We can identify the initial level with:

type Ancestor = keyof typeof offspring; // Alex|Sarah

But how do we pinpoint the exact names for Parent (Alex, Sarah) and Child (Ella, Fiona, Hannah, Isabella)?

typescript types

Answer 1

Answer №1

If your goal is to extract each literal value, you can achieve it by following this approach:

const grandchildren = {
  Karen: {
    Ava: ['Alice', 'Amelia'],
    Emma: ['Sarah'],
  },
  Mary: {
    Sophia: ['Grace'],
  },
} as const;

type Grandmother = keyof typeof grandchildren;

type Mother = {
  [K in Grandmother]: keyof typeof grandchildren[K];
}[Grandmother];

type Kid = {
  [K in Grandmother]: {
    [M in Mother]: typeof grandchildren[K][M extends keyof typeof grandchildren[K] ? M : never];
  };
}[Grandmother][Mother][number];

To obtain the keys for Mother, simply extract the attribute keys from grandchildren

For Kid, extract and flatten the arrays associated with each mother under each grandmother using [number]

The condition

M extends keyof typeof grandchildren[K] ? M : never

is necessary because at this point M could be a key of either 'Karen' or 'Mary', making TypeScript unsure if M can index grandchildren[K].

I hope this explanation proves helpful.

Answer 2

If your goal is to extract each literal value, you can achieve it by following this approach:

const grandchildren = {
  Karen: {
    Ava: ['Alice', 'Amelia'],
    Emma: ['Sarah'],
  },
  Mary: {
    Sophia: ['Grace'],
  },
} as const;

type Grandmother = keyof typeof grandchildren;

type Mother = {
  [K in Grandmother]: keyof typeof grandchildren[K];
}[Grandmother];

type Kid = {
  [K in Grandmother]: {
    [M in Mother]: typeof grandchildren[K][M extends keyof typeof grandchildren[K] ? M : never];
  };
}[Grandmother][Mother][number];

To obtain the keys for Mother, simply extract the attribute keys from grandchildren

For Kid, extract and flatten the arrays associated with each mother under each grandmother using [number]

The condition

M extends keyof typeof grandchildren[K] ? M : never

is necessary because at this point M could be a key of either 'Karen' or 'Mary', making TypeScript unsure if M can index grandchildren[K].

I hope this explanation proves helpful.

In what way can I utilize const assertions to extract literal types from deeply nested objects?

Answer №1

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