从数组值推断类型

Question

Using Typescript 3.8.3.

I'm trying to set up types for the following data, but cannot for the life of me figure it out.

type Info = {
  title: string;
  description: string;
    items: Array<Item<object>>;
}

type Item<T> = {
  title: string;
  data: T;
}

const info: Info = {
  title: 'some title',
  description: 'some description',
  items: [
    { title: 'title1', data: { param1: 'something', param2: 'something else' } },
    { title: 'title2', data: { param3: 'abc' } },
    { title: 'title3', data: { param1: 'not the same as above, just the key name', param4: 123 } }
  ]
};

info.items[0].data.param1 // Property 'param1' does not exist on type 'object'

I know Array<Item<object>> is wrong, but I can't figure out how I can infer type of the union of an array of elements.

Is what I want to achieve even possible?

Answer 1

The only thing you can do is:

type Info = {
  title: string;
  description: string;
    items: ReadonlyArray<Item<object>>; // Only work with ReadonlyArray
}

type Item<T> = {
  title: string;
  data: T;
}

const info = {
  title: 'some title',
  description: 'some description',
  items: [
    { title: 'title1', data: { param1: 'something', param2: 'something else' } },
    { title: 'title2', data: { param3: 'abc' } },
    { title: 'title3', data: { param1: 'not the same as above, just the key name', param4: 123 } }
  ]
} as const;

info.items[0].data.param1 // 'something' as info type is known
const anotherInfo: Info = info // info const be assigned to Info

If info is not 'const' type, you can modify it like: info.items = [] , so typescript cannot guarantee the value / type of info.items[0].data.param1

Answer 2

Would this work?

type Info<T> = {
  title: string;
  description: string;
  items: Item<T>[];
}

type Item<T> = {
  title: string;
  data: T;
}

const info = {
  title: 'some title',
  description: 'some description',
  items: [
    { title: 'title1', data: { param1: 'something', param2: 'something else' } },
    { title: 'title2', data: { param3: 'abc' } },
    { title: 'title3', data: { param1: 'not the same as above, just the key name', param4: 123 } }
  ]
};

info.items[0].data.param1 // string | undefined