narrowing

[ashen spindle]

how can i make sure i have a type that is an item that exists in the array. this works, but i have to make sure the array is narrowed. how can i ensure arrays are narrowed whenever i use this?

type Extract<T extends string[]> = {
  array: T;
  elem: T[number];
}

[zenith steeple]

Show some more code, a playground reproduction would be best.

[ashen spindle]

https://www.typescriptlang.org/play?ssl=24&ssc=14&pln=1&pc=1#code/C4TwDgpgBAKhDOwA8MoQB7AgOwCbykQCcBLbAcwG0BdAPigF4oBvAKCigEMijOQAuWAG52aADYQAtoJiVsAV0kAjCEWoiAvq20AzedgDGwEgHtsUMKWzJUGLHgLEyVOgAosiGQhu0AlC1EDM3gTCQA6MRNyd29fTW0g7EQoD2BGAI5uXgEoSgAiAAsIMUi86gAaUWKpQTyARjzKrVZLMmAYxDjtVkTk1IAmdLZMnj5BfKKSkzKuAl7gSo5q6Sh6xtZm1usO4H64oA/

1st print no error. 2nd print there is an error because of narrowing. how do i ensure i always have the 2nd one where elem can only be an element in array

[zenith steeple]

That's expected behavior, your test2 now doesn't fit the Test type.

[ashen spindle]

how can i ensure test becomes like test2 with always having to specify as const

is there anything i can do in the type

test is bad because i can use any string for the elem

[zenith steeple]

So you want your elem can only be values from array?

[ashen spindle]

yes

[zenith steeple]

I don't see an ergonomic way to do it if you want to avoid manually type annotating things, other than using a factory function.

[tame solstice]

Burrito#6903

Playground Link

Preview:```ts
type Test<T extends string[]> = {
array: T;
elem: T[number];
}

function print<T extends string[]>(test: Test<T>) {
console.log(test);
}

function createTest<const T extends string>(array: T[], elem: T & {}) {
return { array, elem }
}

const test1 = createTest(['hello'], '1')
...```

You can choose specific lines to embed by selecting them before copying the link.

[ashen spindle]

ok ty

is what i have still okay to use if i make sure my array types are narrowed?

my other option is just throwing the generic out and just using the type. so string[] and string

[zenith steeple]

🤷‍♂️

Depends on how much you care about the correctness of "elem must be in array"