Is it possible to define a type for a nested object with variable depth?

Question

I have a function that nests objects based on the length of an input array. E.g.:

fn(['a']) -> Record<string, string>

fn(['a', 'b']) -> Record<Record<string, string>>

I'm defining the return type as:

type Ret = {
  [k: string]: string | Ret;
}

However, this doesn't know the depth of the object. If the input as a TS tuple, then in theory it should be possible to know the depth of the return type. Is this currently possible?

AFAIK TypeScript support for dependent types is limited. Your Ret type would need a "depth" parameter that is reduced. So you'd have Ret<0> = string and Ret<n> = {[k: string]: Ret<n-1>}, obviously this syntax is made up. Maybe this article, or this one may help giving you some ideas — GACy20
– GACy20, Commented Jun 16, 2021 at 12:50

kaya3 · Accepted Answer · 2021-06-16 12:58:41Z

4

Using a recursively defined conditional type:

type NestedRecord<T extends any[]>
    = T extends [any, ...infer R]
    ? Record<string, NestedRecord<R>>
    : string

Example:

type Test = NestedRecord<['a', 'b']>
// {[x: string]: Record<string, string>}

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answered Jun 16, 2021 at 12:58

kaya3

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