Is it possible to define a type from an array object in typescript

Question

I have an array

const a = [
  {
     name: 'n1',
     text: 't1',
  },
  {
     name: 'n2',
     text: 't2',
  },
  {
     name: 'n3',
     text: 't3',
  },
]

I want to define a type type t = 'n1' | 'n2' | 'n3' from array a, is it possible in typescript?

Nick · Accepted Answer · 2021-06-04 01:52:58Z

4

If you define your array with a const assertion (as const) then you can use typeof a[number]["name"]:

const a = [
  {
     name: 'n1',
     text: 't1',
  },
  {
     name: 'n2',
     text: 't2',
  },
  {
     name: 'n3',
     text: 't3',
  },
] as const;

type Names = typeof a[number]["name"];

Here is a working repl with the type definition.

answered Jun 4, 2021 at 1:52

Nick

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1 Comment

Gisheri Over a year ago

Is there a way to apply 'as const' after the fact, like on the same statement as the typeof ( it doesn't seem to work, but I'm wondering if there is a workaround for that if you can't modify the original data) ?

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