Typescript: How can I cast a variable as an element of an array?

Question

Array:

const myStrings = ["one", "two", "three"];
const newString = "two";

newString will simply be of type string, but I want the type to be something like element of myStrings.

How can I do this if the values in the array may or may not change?

I think you want type literals but it's not clear. What does something like element of myStrings mean? — Liam
– Liam, Commented Apr 7, 2022 at 9:35
Well, ALL VOTES are subjective. "This question does not show any research effort; it is unclear or not useful". This seems to tick (as a minimum) two of those boxes. ¯\_(ツ)_/¯ — Liam
– Liam, Commented Apr 7, 2022 at 9:55

T.J. Crowder · Accepted Answer · 2022-04-07 09:57:11Z

1

You can use typeof myStrings[number], but only if myStrings has as const telling TypeScript that its contents don't change:

const myStrings = ["one", "two", "three"] as const;
type MyStringsElement = typeof myStrings[number]; // For convenience
const newString: MyStringsElement = "two";

Playground example

With the myStrings contents you've shown, the type of newString will be "one" | "two" | "three". That's a union of three string literal types, meaning that the only valid values for newString are "one", "two', or "three" (as compile-time constant values).

(Note: typeof myStrings[number] is evaluated by TypeScript as (typeof myStrings)[number], which may be surprising. I used to think I needed the parentheses, and I'm not sure it's not best to have them for clarity even if they aren't needed... :-) )

edited Apr 7, 2022 at 9:57

answered Apr 7, 2022 at 9:39

T.J. Crowder

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