Clojure's core/partition_by in TypeScript

✓ Verified: Clojure 1.12

How to use

Install via yarn add locutus and import: import { partition_by } from 'locutus/clojure/core/partition_by'.

Or with CommonJS: const { partition_by } = require('locutus/clojure/core/partition_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	`partition_by((value) => value % 2 === 0, [1, 1, 2, 2, 3, 4, 5, 5])`	`[[1, 1], [2, 2], [3], [4], [5, 5]]`
2	`partition_by((value) => value.length, ['a', 'ab', 'cd', 'efg'])`	`[['a'], ['ab', 'cd'], ['efg']]`
3	`partition_by((value) => value, [])`	`[]`

Notes

Groups consecutive values while selector(value) remains equal, similar to Clojure partition-by.

Here's what our current TypeScript equivalent to Clojure's core/partition_by looks like.

type PartitionBySelector<T, TKey> = (value: T, index: number, array: T[]) => TKey

function stablePartitionKey(value: unknown): string {
  if (value === null) {
    return 'null'
  }
  if (typeof value === 'undefined') {
    return 'undefined'
  }
  if (typeof value === 'number') {
    if (Number.isNaN(value)) {
      return 'number:NaN'
    }
    if (Object.is(value, -0)) {
      return 'number:-0'
    }
    return `number:${value}`
  }
  if (typeof value === 'string') {
    return `string:${value}`
  }
  if (typeof value === 'boolean') {
    return `boolean:${value}`
  }
  if (typeof value === 'bigint') {
    return `bigint:${value}`
  }
  if (Array.isArray(value)) {
    return `array:[${value.map((item) => stablePartitionKey(item)).join(',')}]`
  }
  if (typeof value === 'object') {
    const record = value as { [key: string]: unknown }
    const entries = Object.keys(record)
      .sort()
      .map((key) => `${JSON.stringify(key)}:${stablePartitionKey(record[key])}`)
    return `object:{${entries.join(',')}}`
  }

  return `${typeof value}:${String(value)}`
}

export function partition_by<T, TKey>(selector: PartitionBySelector<T, TKey>, values: T[] | unknown): T[][] {
  //      discuss at: https://locutus.io/clojure/partition_by/
  // parity verified: Clojure 1.12
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Groups consecutive values while selector(value) remains equal, similar to Clojure partition-by.
  //       example 1: partition_by((value) => value % 2 === 0, [1, 1, 2, 2, 3, 4, 5, 5])
  //       returns 1: [[1, 1], [2, 2], [3], [4], [5, 5]]
  //       example 2: partition_by((value) => value.length, ['a', 'ab', 'cd', 'efg'])
  //       returns 2: [['a'], ['ab', 'cd'], ['efg']]
  //       example 3: partition_by((value) => value, [])
  //       returns 3: []

  if (typeof selector !== 'function') {
    throw new TypeError('partition_by(): selector must be a function')
  }

  if (!Array.isArray(values) || values.length === 0) {
    return []
  }

  const out: T[][] = []
  let currentBucket: T[] = []
  let previousKey: string | null = null

  for (let i = 0; i < values.length; i++) {
    const value = values[i] as T
    const nextKey = stablePartitionKey(selector(value, i, values))

    if (previousKey === null || nextKey === previousKey) {
      currentBucket.push(value)
      previousKey = nextKey
      continue
    }

    out.push(currentBucket)
    currentBucket = [value]
    previousKey = nextKey
  }

  if (currentBucket.length > 0) {
    out.push(currentBucket)
  }

  return out
}

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