Improve string split return type of first array index

[zardoy]

lib Update Request

Please, consider improving user experience for "".split(...) in cases when string literal is passed, which I believe most users do e.g. split(' ') or split('\n')

Configuration Check

My compilation target is ES2015 and my lib is the default.

Missing / Incorrect Definition

TypeScript/src/lib/es5.d.ts

Line 460 in b70784e

split(separator: string | RegExp, limit?: number): string[];

Essentially duplicate of #49635, but with #49635 (comment) handled:

split<T extends string | RegExp>(separator: T, limit?: number): T extends `${string}${infer U}` ? [string, ...string[]] : string[];

And this should added to param JSDoc then: Only passing `""` and `new RegExp("")` values can result in returning an empty array.

Sample Code

This is super inconvenient when used with destructure:

const [firstLine, ...restLine] = str.split('\n')
// firstLine is possibly undefined, need add exclamation marks everywhere or `as [string, ...string]` above

I also wanted to do the same for when regexp literal is passed e.g. split(/\n\r?/) as only split(new RegExp('')) can result in empty array (if I'm not mistaken), which I believe no one does, but the literal value can't be captured anyway

Documentation Link

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/split#description (note)

P.S. I remembered of this issue after #49682, which significantly improved type-checking experience by using less exclamation marks

[RyanCavanaugh]

FWIW: we have a pretty low appetite for making the standard library definitions extremely complex for the sake of NUIA, which is a flag we added with significant hesitation because it makes things like these difficult to handle.

[EzioMercer]

Original

I came up with more strict overloads when limit is 0 and when separator is string:

type Split<Source extends string, Separator extends string> = Source extends `${infer Left}${Separator}${infer Right}`
    ? [...(string extends Left ? string[] : [`${Left}`]), ...Split<Right, Separator>]
    : Source extends ""
      ? Separator extends ""
          ? []
          : [""]
      : string extends Source
        ? string[]
        : [Source];

interface String {
    split<Source extends string, Separator extends string>(
        this: Source,
        separator: Separator,
        limit?: undefined,
    ): Split<Source, Separator>;

    split(separator: string | RegExp, limit: 0): [];
}

You can see types in action on playground (with examples)

[zardoy]

Good that we have any activity there, @EzioMercer can you open your version of pr?;)

The issue is still really annoying and I wish it could be fixed:(

[EzioMercer]

@zardoy I will try. Why did you dislike my issue?)