Constraining jsdoc/typescript type to an array member

Question

I'm using typescript in jsdoc, and am trying to constrain a variable to one of a known set of values that I have in an array.

I know I can do it like this:

/** @type {'one'|'two'|'three'} */
let v = 'four';
// ==> Error, type 'four' is not assignable to type 'one'|'two'|'three'

In my case, I have the desired values nearby in an array. To avoid retyping, I'd like to somehow reference them, but I don't know if it's possible. I'd like something like this:

const OPTIONS = ['one', 'two', 'three'];

/** @type {string<Options>} */
let v = 'four';
// ==> Desired -- Error, type 'four' is not assignable to type 'one'|'two'|'three'
// ==> but that doesn't actually work...

Is there some way to do this?

mustofa.id · Accepted Answer · 2022-07-09 15:08:15Z

8

As Augusts 28, 2021, It's possible using JSDoc inline @type {const} like following:

const OPTIONS = /** @type {const} */ (['one', 'two', 'three']);

/** @type {typeof OPTIONS[number]} */
let v = 'four'; 
//  ^ error: Type '"four"' is not assignable to type '"one" | "two" | "three" |

answered Jul 9, 2022 at 15:08

mustofa.id

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Lesiak · Accepted Answer · 2020-04-28 16:58:29Z

3

I don't think you can achieve that with an array, as they are mutable in runtime:

const OPTIONS = ['one', 'two', 'three'];
OPTIONS[0] = 'BOOM';

You can, however, change an array to a tuple (tuples are immutable):

const OPTIONS = ['one', 'two', 'three'];                
const OPTIONS_TUPLE = ['one', 'two', 'three'] as const;

Compare inferred types:

// const OPTIONS: string[]
// const OPTIONS_TUPLE: readonly ["one", "two", "three"]

Now, you can retrieve the type you want:

const OPTIONS_TUPLE = ['one', 'two', 'three'] as const;
type OptionsValue = typeof OPTIONS_TUPLE[number];
const x: OptionsValue = 'four'; 
//TS2322: Type '"four"' is not assignable to type '"one" | "two" | "three"'.

answered Apr 28, 2020 at 16:58

Lesiak

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1 Comment

Benj Over a year ago

Thanks, really helpful. Unfortunately it looks like as const is not currently available in jsdoc syntax, but i'll keep an eye on this for down the road. github.com/Microsoft/TypeScript/issues/30445 Leaving question open for a while in case there are other suggestions that are currently possible.

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