fix: add the type readonly array to the partition

[ssi02014]

Hello 👋 Here's a suggestion.
Currently, it does not work correctly for arrays with const assertion applied.

I think it would be nice to add a readonly T[] type, what do you think?

AS-IS

function partition<unknown>(
  arr: unknown[],
  isInTruthy: (value: unknown) => boolean
): [truthy: unknown[], falsy: unknown[]];

function partition<unknown>(
  arr: unknown[],
  isInTruthy: (value: unknown) => boolean
): [truthy: unknown[], falsy: unknown[]];

TO-BE

const arr = [1, 2, 3, 4, 5] as const;

const [evnes, odds] = partition(arr, (item) => item % 2 === 0);
// const evens: (1 | 2 | 3 | 4 | 5)[]
// const odds: (1 | 2 | 3 | 4 | 5)[]

const arr = [
  { name: "mj", age: 27 },
  { name: "gromit", age: 30 },
] as const;

const [ob, yb] = partition(arr, (item) => item.age >= 30);
/*
  const ob: ({
    readonly name: "mj";
    readonly age: 27;
  } | {
    readonly name: "gromit";
    readonly age: 30;
  })[]

 const yb ({
    readonly name: "mj";
    readonly age: 27;
  } | {
    readonly name: "gromit";
    readonly age: 30;
  })[]
*/

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[ssi02014]

I think this readonly T[] type should be added to other array utility functions as well.
I can work on it incrementally if it makes sense and is needed. 🤗

[raon0211]

Hi @ssi02014, thanks for your contribution!

As you mentioned, our library does not currently support readonly arrays.

Meanwhile, This would affect a large portion of our function interfaces, and I think using readonly types in TypeScript is a minor use case.

Because of our design principles which value simplicity, I think this would not meet our design principle.

What do you think?

[ssi02014]

@raon0211 Thank you for your feedback 🙏

In my case, I wanted to narrow down the types as much as possible in my development, so I was disappointed to see a type error when using a readonly array.

Because of our design principles which value simplicity, I think this would not meet our design principle.

The readonly array can be used in more places than you might think, and I didn't think adding that type broke the principle of simplicity, but that might be a matter of perspective.

I think it's worth considering whether it's more appropriate to keep the existing principle and preclude users from using readonly, or to add the readonly type but change the function interface.

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[raon0211]

How about this?

Since T[] can be assigned to readonly T[], we can change all our array utilities to accept only readonly T[].

For example, the signature of partition would become:

function partition<T>(arr: readonly T[], isInTruthy: (value: T) => boolean): [truthy: T[], falsy: T[]];

However, I'd like our documentation to state that partition accepts T[] as its argument (not readonly). This is because many of our users might get confused if they see that partition only accepts readonly arrays.

[ssi02014]

@raon0211
Yes. I like your suggestion as well, as it fulfills my initial purpose of "readonly array are compatible".

But, How do you feel about documentation and code being out of sync, which can lead to confusion?

I will follow the direction of the es-toolkit maintainers! 🤗

[raon0211]

I think for the sake of simplicity, the docs and implementation could go a little bit out of sync.

[ssi02014]

@raon0211 I've reverted the document, annotations I was working on. thank you! 🙏

[raon0211]

Thanks!