Derive Type from Array Typescript

Question

I have an array

const frequencyOptions = ['Daily', 'Weekly', 'Monthly'];

I need to set Typescript Type to another array that can contain values from the options array only like

let frequencyValues = ['Daily', 'Weekly'] // can be any combination of options array

I need to set the typescript type of the values array so that it can only accept values that are only available in the frequencyOptions array. — Biswa
– Biswa, Commented Oct 31, 2021 at 8:25
@AlekseyL. Yes, the frequencyOptions array will be mostly constant/static — Biswa
– Biswa, Commented Oct 31, 2021 at 8:26
Does this answer your question? TypeScript: Define a union type from an array of strings — Tino
– Tino, Commented Oct 31, 2021 at 8:37

Aleksey L. · Accepted Answer · 2021-10-31 08:27:19Z

3

If you're controlling options array creation you can:

const frequencyOptions = ['Daily', 'Weekly', 'Monthly'] as const;
type Option = typeof frequencyOptions[number]; // "Daily" | "Weekly" | "Monthly"

let frequencyValues: Option[] = ['Daily', 'Weekly'];

// Expect error
frequencyValues = ['foo'] // Type '"foo"' is not assignable to type '"Daily" | "Weekly" | "Monthly"'.

Playground

as const prevents literal types widening to string, then we query tuple item type.

answered Oct 31, 2021 at 8:27

Aleksey L.

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