🚨 Array .map() type too wide

[regnaio]

🔎 Search Terms

TypeScript Playground Link

🕗 Version & Regression Information

Unknown

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYewdgzgLgBACgQQJICUDKMC8MDaAoGQ3AcgENiAaGABgF0KCidiAjSmARnscOeHYBM3WjFIQYoSFADcePJOgwAwgHkAckoQAVAKJrtOgCJZ4ydADoAtqQAOACgBupADYBXAKZYAfDAAGAEgBvJzd3HDoAXwBaIJCPHC4I31FxBSgASlk8KABPG08lcGBSKHcwEvcAExM7XPyQADNldU1dfV1DdJwwV0sWdwAnWlkgA

💻 Code

const PAIRS = [
    ['a', 0],
    ['b', 1],
    ['c', 2],
] as const;

const CONCATENATED = PAIRS.map(value => `${value[0]}-${value[1]}` as const);

type Concatenated = (typeof CONCATENATED)[number];

🙁 Actual behavior

If you hover over Concatenated, you can see that its type is type Concatenated = "a-0" | "a-2" | "a-1" | "b-0" | "b-2" | "b-1" | "c-0" | "c-2" | "c-1".

🙂 Expected behavior

The expected type should instead be type Concatenated = "a-0" | "b-1" | "c-2".

Additional information about the issue

No response

[jcalz]

TS doesn't have direct support for correlated unions as described in #30581. The only support is for generics of the form described at #47109, which is kind of yucky for a case like this:

type PairFor = { [T in typeof PAIRS[number] as T[0]]: T[1] }
type MapRet<K extends keyof PairFor> =
    { [P in K]: `${P}-${PairFor[P]}` }[K]

const CONCATENATED =
    PAIRS.map(<K extends keyof PairFor>(
        value: readonly [K, PairFor[K]]
    ): MapRet<K> => `${value[0]}-${value[1]}` as const);

type Concatenated = (typeof CONCATENATED)[number];
// type Concatenated = "a-0" | "b-1" | "c-2"

Playground link

I don't know if this is considered a duplicate of #30581 or a design limitation.

(edit: 🚨❓ 🚑🚔🚒)

[regnaio]

Thank you so much for your detailed help, @jcalz! 😄

I initially tried to get the result "a-0" | "b-1" | "c-2" with another Playground, though I can see why this other approach gives the wider result "a-0" | "a-2" | "a-1" | "b-0" | "b-2" | "b-1" | "c-0" | "c-2" | "c-1". Is there a way to get the narrower result along the lines of this approach? Or would the solution you shared above be the simplest way to achieve this?

Thank you again!