Kotlin's collections.groupBy in TypeScript

How to use

Install via yarn add locutus and import: import { groupBy } from 'locutus/kotlin/collections/groupBy'.

Or with CommonJS: const { groupBy } = require('locutus/kotlin/collections/groupBy')

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	groupBy([1, 2, 3, 4], (n) => (n % 2 === 0 ? 'even' : 'odd'))	{odd: [1, 3], even: [2, 4]}
2	groupBy(['ant', 'bear', 'cat'], (s) => s.length)	{'3': ['ant', 'cat'], '4': ['bear']}
3	groupBy([], () => 'x')	{}

Notes

• Groups elements using selector(value), similar to Kotlin’s groupBy.

Here's what our current TypeScript equivalent to Kotlin's collections.groupBy looks like.

TSTypeScript JSJavaScript TSStandalone TS JSStandalone JS

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export function groupBy<T>(
  values: T[] | unknown,
  selector: (value: T, index: number, array: T[]) => string | number | symbol,
): Record<string, T[]> {
  //  discuss at: https://locutus.io/kotlin/collections/groupBy/
  // original by: Kevin van Zonneveld (https://kvz.io)
  //      note 1: Groups elements using selector(value), similar to Kotlin's groupBy.
  //   example 1: groupBy([1, 2, 3, 4], (n) => (n % 2 === 0 ? 'even' : 'odd'))
  //   returns 1: {odd: [1, 3], even: [2, 4]}
  //   example 2: groupBy(['ant', 'bear', 'cat'], (s) => s.length)
  //   returns 2: {'3': ['ant', 'cat'], '4': ['bear']}
  //   example 3: groupBy([], () => 'x')
  //   returns 3: {}

  if (!Array.isArray(values) || typeof selector !== 'function') {
    return {}
  }

  const out: Record<string, T[]> = {}
  for (let i = 0; i < values.length; i++) {
    const value = values[i] as T
    const key = String(selector(value, i, values))
    const bucket = out[key]
    if (bucket) {
      bucket.push(value)
    } else {
      out[key] = [value]
    }
  }

  return out
}

export function groupBy(values, selector) {
  //  discuss at: https://locutus.io/kotlin/collections/groupBy/
  // original by: Kevin van Zonneveld (https://kvz.io)
  //      note 1: Groups elements using selector(value), similar to Kotlin's groupBy.
  //   example 1: groupBy([1, 2, 3, 4], (n) => (n % 2 === 0 ? 'even' : 'odd'))
  //   returns 1: {odd: [1, 3], even: [2, 4]}
  //   example 2: groupBy(['ant', 'bear', 'cat'], (s) => s.length)
  //   returns 2: {'3': ['ant', 'cat'], '4': ['bear']}
  //   example 3: groupBy([], () => 'x')
  //   returns 3: {}

  if (!Array.isArray(values) || typeof selector !== 'function') {
    return {}
  }

  const out = {}
  for (let i = 0; i < values.length; i++) {
    const value = values[i]
    const key = String(selector(value, i, values))
    const bucket = out[key]
    if (bucket) {
      bucket.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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