How can I get type from object of array

Question

type Item = {
  id: string;
  value: string;
};

const items: Readonly<Item>[] = [
  { id: 'id1', value: 'TODO1' },
  { id: 'id2', value: 'TODO2' },
  { id: 'id3', value: 'TODO3' },
];

I want to get type 'TODO1' | 'TODO2' | 'TODO3';

const items = [...] as const;

type Type = typeof items[number]['value'];

I could get a type by const assertion. But this case, I lost my items type..

I don't think you should need the Item type at all - TS will infer it properly. There shouldn't be any issues having a narrow type when a wide one is accepted too. Remove : Readonly<Item>[] and use as const — CertainPerformance
– CertainPerformance, Commented Jun 22, 2021 at 3:41

CertainPerformance · Accepted Answer · 2021-06-22 03:44:47Z

1

You could define the array as const initially, then assign it to items later:

const itemsInitial = [
  { id: 'id1', value: 'TODO1' } as const,
  { id: 'id2', value: 'TODO2' } as const,
  { id: 'id3', value: 'TODO3' } as const,
];
type Type = typeof itemsInitial[number]['value'];
const items: Readonly<Item>[] = itemsInitial;

But you may not need the Item type at all - I don't see a need for it yet, or a need for the Readonly<Item>[] type, given the code in the question.

answered Jun 22, 2021 at 3:44

CertainPerformance

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