Typescript - How Do I Get A Typed Array?

Question

My Sample use case.

type someEnum = 'a' | 'b';

const someObj: { [K in someEnum]: string } = {
    a: 'a',
    b: 'b',
};

const a: Array<someEnum> = ['a', 'b'];

// In Future update someEnum

type someEnum = 'a' | 'b' | 'c';

// Gives an error;

// Property 'c' is missing in type '{ a: string; b: string; }' 
// but required in type '{ a: string; b: string; c: string; }'.

const someObj: { [K in someEnum]: string } = {
    a: 'a',
    b: 'b',
};

// No error;
const a: Array<someEnum> = ['a', 'b'];

// Is There someThing like;

// const typedArray: [K in someEnum]

It would be really cool to have such a feature; Is there some functionality to achieve the same. Thanks in advance.

Is it type typedArray = Array<someEnum>; - or what feature are you missing? — GOTO 0
– GOTO 0, Commented Aug 2, 2019 at 9:53

Titian Cernicova-Dragomir · Accepted Answer · 2019-08-02 10:14:31Z

It's tempting to use some workaround to get tuple to union, please don't use it.

You can work around this issue in several ways, the simplest way is if the enum is under your control, start from the array instead of the enum:

const allPosibilitieForSomeEnum = ['a',  'b' , 'c'] as const
type someEnum = typeof allPosibilitieForSomeEnum[number];

You could use an object instead and use keys to get the keys.

Or you could use a function that validates that all memebers are present:

type someEnum = 'a' | 'b' | "c";

function checkEnum<TEnum>() {
    return function <TActual extends TEnum[]>(...p: TActual & ([TEnum] extends [TActual[number]] ? {} : {
        error: ["Array does not contain all members, expected: ", TEnum, "found:", TActual[number]]
    })) {
        return p
    }
}

const someObj =  checkEnum<someEnum>()("a", "b", "c") 
const someObj2 =  checkEnum<someEnum>()("a", "b") ///  Property 'error' is missing in type '["a", "b"]' but required in type '{ error: ["Array does not contain all members, expected: ", someEnum, "found:", "a" | "b"]; }'.

This seems to be the most viable approach, but i guess we cannot index the enum as in we cannot do allPosibilitieForSomeEnum.a

pascalpuetz · Accepted Answer · 2019-08-02 09:45:26Z

0

You want to create a tuple out of a union type. That is indeed possible but a little bit more complex. There is also an issue in the TypeScript Github repository that adresses this.

One of the answers there seems to work pretty well. Here the example in TypeScript Playground.

edited Aug 2, 2019 at 9:45

answered Aug 2, 2019 at 9:39

pascalpuetz

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3 Comments

Titian Cernicova-Dragomir Over a year ago

Please, please make sure you udenrstand what you are getting into before using this: github.com/microsoft/TypeScript/issues/…

Amol Gupta Over a year ago

Is'nt there a 'cleaner' way of doing this ?

pascalpuetz Over a year ago

No, currently there is no cleaner way - except for doing this the other way around. Creating the tuple type yourself and infer the union type.

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