Elixir's Enum.scan in TypeScript

How to use

Install via yarn add locutus and import: import { scan } from 'locutus/elixir/Enum/scan'.

Or with CommonJS: const { scan } = require('locutus/elixir/Enum/scan')

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	scan([1, 2, 3, 4], (acc, value) => Number(acc) + Number(value))	[1, 3, 6, 10]
2	scan([1, 2, 3], 10, (acc, value) => Number(acc) + Number(value))	[11, 13, 16]
3	scan([], 5, (acc, value) => Number(acc) + Number(value))	[]

Notes

• Produces running accumulator states, similar to Elixir Enum.scan/2 and Enum.scan/3.

Here's what our current TypeScript equivalent to Elixir's Enum.scan looks like.

TSTypeScript JSJavaScript TSStandalone TS JSStandalone JS

Copy code

type ScanReducer<TAccumulator, TValue> = (
  accumulator: TAccumulator,
  value: TValue,
  index: number,
  array: TValue[],
) => TAccumulator

function isScanReducer<TAccumulator, TValue>(value: unknown): value is ScanReducer<TAccumulator, TValue> {
  return typeof value === 'function'
}

export function scan<TValue>(values: TValue[] | unknown, reducer: ScanReducer<TValue, TValue>): TValue[]
export function scan<TAccumulator, TValue>(
  values: TValue[] | unknown,
  initial: TAccumulator,
  reducer: ScanReducer<TAccumulator, TValue>,
): TAccumulator[]
export function scan<TAccumulator, TValue>(
  values: TValue[] | unknown,
  initialOrReducer: TAccumulator | ScanReducer<TValue, TValue>,
  maybeReducer?: ScanReducer<TAccumulator, TValue>,
): Array<TAccumulator | TValue> {
  //  discuss at: https://locutus.io/elixir/scan/
  // original by: Kevin van Zonneveld (https://kvz.io)
  //      note 1: Produces running accumulator states, similar to Elixir Enum.scan/2 and Enum.scan/3.
  //   example 1: scan([1, 2, 3, 4], (acc, value) => Number(acc) + Number(value))
  //   returns 1: [1, 3, 6, 10]
  //   example 2: scan([1, 2, 3], 10, (acc, value) => Number(acc) + Number(value))
  //   returns 2: [11, 13, 16]
  //   example 3: scan([], 5, (acc, value) => Number(acc) + Number(value))
  //   returns 3: []

  if (!Array.isArray(values)) {
    return []
  }

  const hasInitial = typeof maybeReducer === 'function'

  if (values.length === 0) {
    return []
  }

  const out: Array<TAccumulator | TValue> = []

  if (hasInitial) {
    const reducer = maybeReducer
    if (!isScanReducer<TAccumulator, TValue>(reducer)) {
      throw new TypeError('scan(): reducer must be a function')
    }

    let accumulator = initialOrReducer as TAccumulator
    for (let i = 0; i < values.length; i++) {
      accumulator = reducer(accumulator, values[i] as TValue, i, values)
      out.push(accumulator)
    }
    return out
  }

  if (!isScanReducer<TValue, TValue>(initialOrReducer)) {
    throw new TypeError('scan(): reducer must be a function')
  }

  const reducer = initialOrReducer
  let accumulator = values[0] as TValue
  out.push(accumulator)
  for (let i = 1; i < values.length; i++) {
    accumulator = reducer(accumulator, values[i] as TValue, i, values)
    out.push(accumulator)
  }

  return out
}

function isScanReducer(value) {
  return typeof value === 'function'
}
export function scan(values, initialOrReducer, maybeReducer) {
  //  discuss at: https://locutus.io/elixir/scan/
  // original by: Kevin van Zonneveld (https://kvz.io)
  //      note 1: Produces running accumulator states, similar to Elixir Enum.scan/2 and Enum.scan/3.
  //   example 1: scan([1, 2, 3, 4], (acc, value) => Number(acc) + Number(value))
  //   returns 1: [1, 3, 6, 10]
  //   example 2: scan([1, 2, 3], 10, (acc, value) => Number(acc) + Number(value))
  //   returns 2: [11, 13, 16]
  //   example 3: scan([], 5, (acc, value) => Number(acc) + Number(value))
  //   returns 3: []

  if (!Array.isArray(values)) {
    return []
  }

  const hasInitial = typeof maybeReducer === 'function'

  if (values.length === 0) {
    return []
  }

  const out = []

  if (hasInitial) {
    const reducer = maybeReducer
    if (!isScanReducer(reducer)) {
      throw new TypeError('scan(): reducer must be a function')
    }

    let accumulator = initialOrReducer
    for (let i = 0; i < values.length; i++) {
      accumulator = reducer(accumulator, values[i], i, values)
      out.push(accumulator)
    }
    return out
  }

  if (!isScanReducer(initialOrReducer)) {
    throw new TypeError('scan(): reducer must be a function')
  }

  const reducer = initialOrReducer
  let accumulator = values[0]
  out.push(accumulator)
  for (let i = 1; i < values.length; i++) {
    accumulator = reducer(accumulator, values[i], i, values)
    out.push(accumulator)
  }

  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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