Ruby's Array.group_by in TypeScript

How to use

Install via yarn add locutus and import: import { group_by } from 'locutus/ruby/Array/group_by'.

Or with CommonJS: const { group_by } = require('locutus/ruby/Array/group_by')

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	group_by(['a', 'b', 'a'])	{a: ['a', 'a'], b: ['b']}
2	group_by([1, 2, 1, 3])	{'1': [1, 1], '2': [2], '3': [3]}
3	group_by([])	{}

Ruby nil and TypeScript/JavaScript undefined

Ruby's nil and TypeScript/JavaScript undefined are semantically equivalent—both represent "no value" or "nothing here". However, they serialize differently: nil becomes null in JSON, while undefined is typically omitted or becomes undefined.

Locutus Ruby functions return undefined (idiomatic TypeScript/JavaScript) where Ruby would return nil. Our parity tests treat these as equivalent when verifying against native Ruby.

Notes

• Groups array elements by identity key, mirroring array.group_by { |v| v }.

Here's what our current TypeScript equivalent to Ruby's Array.group_by looks like.

TSTypeScript JSJavaScript TSStandalone TS JSStandalone JS

Copy code

export function group_by<T>(arr: T[] | unknown): Record<string, T[]> {
  // parity verified: Ruby 3.3
  //      discuss at: https://locutus.io/ruby/Array/group_by/
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Groups array elements by identity key, mirroring `array.group_by { |v| v }`.
  //       example 1: group_by(['a', 'b', 'a'])
  //       returns 1: {a: ['a', 'a'], b: ['b']}
  //       example 2: group_by([1, 2, 1, 3])
  //       returns 2: {'1': [1, 1], '2': [2], '3': [3]}
  //       example 3: group_by([])
  //       returns 3: {}

  if (!Array.isArray(arr)) {
    return {}
  }

  const out: Record<string, T[]> = {}

  for (const value of arr) {
    const key = String(value)
    const existing = out[key]
    if (existing) {
      existing.push(value)
    } else {
      out[key] = [value]
    }
  }

  return out
}

export function group_by(arr) {
  // parity verified: Ruby 3.3
  //      discuss at: https://locutus.io/ruby/Array/group_by/
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Groups array elements by identity key, mirroring `array.group_by { |v| v }`.
  //       example 1: group_by(['a', 'b', 'a'])
  //       returns 1: {a: ['a', 'a'], b: ['b']}
  //       example 2: group_by([1, 2, 1, 3])
  //       returns 2: {'1': [1, 1], '2': [2], '3': [3]}
  //       example 3: group_by([])
  //       returns 3: {}

  if (!Array.isArray(arr)) {
    return {}
  }

  const out = {}

  for (const value of arr) {
    const key = String(value)
    const existing = out[key]
    if (existing) {
      existing.push(value)
    } else {
      out[key] = [value]
    }
  }

  return out
}

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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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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