Rust's str::rsplit_once in TypeScript

How to use

Install via yarn add locutus and import: import { rsplit_once } from 'locutus/rust/str/rsplit_once'.

Or with CommonJS: const { rsplit_once } = require('locutus/rust/str/rsplit_once')

Use a bundler that supports tree-shaking so you only ship the functions you actually use. Vite, webpack, Rollup, and Parcel all handle this. For server-side use this is less of a concern.

Examples

These examples are extracted from test cases that automatically verify our functions against their native counterparts.

#	code	expected result
1	rsplit_once('=', 'a=b=c')	['a=b', 'c']
2	rsplit_once('::', 'x::y::z')	['x::y', 'z']
3	rsplit_once(':', 'abc')	null

Notes

• Splits string on the last delimiter occurrence and returns [left, right] or null.

Here's what our current TypeScript equivalent to Rust's str::rsplit_once looks like.

TSTypeScript JSJavaScript TSStandalone TS JSStandalone JS

Copy code

export function rsplit_once(delimiter: string, str: string): [string, string] | null {
  //      discuss at: https://locutus.io/rust/rsplit_once/
  // parity verified: Rust 1.85
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Splits string on the last delimiter occurrence and returns [left, right] or null.
  //       example 1: rsplit_once('=', 'a=b=c')
  //       returns 1: ['a=b', 'c']
  //       example 2: rsplit_once('::', 'x::y::z')
  //       returns 2: ['x::y', 'z']
  //       example 3: rsplit_once(':', 'abc')
  //       returns 3: null

  const source = String(str)
  const value = String(delimiter)

  if (value === '') {
    return [source, '']
  }

  const index = source.lastIndexOf(value)
  if (index < 0) {
    return null
  }

  return [source.slice(0, index), source.slice(index + value.length)]
}

export function rsplit_once(delimiter, str) {
  //      discuss at: https://locutus.io/rust/rsplit_once/
  // parity verified: Rust 1.85
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Splits string on the last delimiter occurrence and returns [left, right] or null.
  //       example 1: rsplit_once('=', 'a=b=c')
  //       returns 1: ['a=b', 'c']
  //       example 2: rsplit_once('::', 'x::y::z')
  //       returns 2: ['x::y', 'z']
  //       example 3: rsplit_once(':', 'abc')
  //       returns 3: null

  const source = String(str)
  const value = String(delimiter)

  if (value === '') {
    return [source, '']
  }

  const index = source.lastIndexOf(value)
  if (index < 0) {
    return null
  }

  return [source.slice(0, index), source.slice(index + value.length)]
}

Improve this function

Locutus is a community effort following The McDonald's Theory: we ship first iterations, hoping others will improve them. If you see something that could be better, we'd love your contribution.

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Got a rainy Sunday afternoon and a taste for a porting puzzle?

• Get inspiration from the Rust std::str docs.
• Click "New file" in the appropriate folder on GitHub. This will fork the project to your account, directly add the file to it, and send a Pull Request to us.

We will then review it. If it's useful to the project and in line with our contributing guidelines your work will become part of Locutus and you'll be automatically credited in the authors section accordingly.

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