获取嵌套值作为类型

Question

Consider following type definition,

type Parent = {
   children: Child[]
}

type Child = {
   label: string
}

const parent: Parent = {
   children: [
       { label: 'label1' },
       { label: 'label2' }
   ]
}

Now, if I want to generate a genetic type, which contains the label values as key and value as any, is it possible to do it with generics? Do note that I am considering only compile time objects only.

type Labels = {
    label1: any,
    label2: any
}

Answer 1

You can define Child and Parent as generics type, where K extends string is used as generic constraint in order to correctly infer the string literal:

type Parent<K extends string> = {
   children: Child<K>[]
}

type Child<K extends string> = {
   label: K
}

type Labels<P extends Parent<any>> = P extends Parent<infer K>
  ? { [Key in K]: string } : never;

Then you can use an helper function in order to automatically infer the label keys from string literals (instead of typing them manually when defining parent as a value):

function Parent<K extends string>(parent: Parent<K>) {
  return parent;
}

const p1 = Parent({
   children: [
       { label: 'label1' },
       { label: 'label2' }
   ]
})

Now everything you need is an helper type that using inference and object mapping, transform the keys in the Labels type:

type Labels<P extends Parent<any>> = P extends Parent<infer K>
  ? { [Key in K]: string } : never;

type Labels1 = Labels<typeof p1>;

// Labels1 now is `{ label1: string; label2: string }`

See it in action @ Playground