Extract a type from a const string array [duplicate]

Question

I know that in Typescript you can do this:

const someOptions = {
  a: "",
  b: "",
  c: ""
} as const 


type SomeType = keyof typeof someOptions

I would like to do something very similar but then using an array as input.

const someOptions = ["a", "b", "c"] as const;
type SomeType = ...

Is this possible?

An array is convenient because I can pass it directly to something like Joi.string().valid(). Also, I also don't have sensible values to define for these things in a key/value object so it feels silly to use an object as starting point.

Haha, did you? It will give you a type with "0" | "1" | "2" | "length" | "push" etc... I need the values not the keys. — Thijs Koerselman
– Thijs Koerselman, Commented Mar 31, 2020 at 7:15
If you do keyof typeof it will, yes. They're the keys of the array type. Your question just shows an ellipsis, though; it's helpful to put what you've tried and what happened. — jonrsharpe
– jonrsharpe, Commented Mar 31, 2020 at 7:16
Thanks, but the ellipsis was meant to indicate that I didn't have a clue :) — Thijs Koerselman
– Thijs Koerselman, Commented Mar 31, 2020 at 7:19

CertainPerformance · Accepted Answer · 2020-03-31 07:13:40Z

6

You can access [number] on the typeof someOptions array to get to the values as a union type:

const someOptions = ["a", "b", "c"] as const;
type SomeType = typeof someOptions[number]
// SomeType is:
// "a" | "b" | "c"

answered Mar 31, 2020 at 7:13

CertainPerformance

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