Flattens an array of integers with arbitrary nesting.

flatten-integer-array.js

// These are just here for unit testing
const assert = require('assert').strict
const expectedError = {
  name: /^TypeError$/,
  message: /not an array|only be integers/
}

/**
 * Flattens an array of arbitrarily nested integer arrays.
 *
 * @param {Array} list - Array of integers or nested integer arrays to any depth
 * @return {Array} The argument flattened
 *
 * @example
 *  flatten([[1, 2, [3]], 4]) -> [1, 2, 3, 4]
 */
const flatten = (list = []) => {
  const flat = []

  if (!Array.isArray(list)) {
    throw new TypeError('Passed argument was not an array.')
  }

  for (entry of list) {
    if (Array.isArray(entry)) {
      flat.push(...flatten(entry))
      continue
    }

    if (!Number.isInteger(entry)) {
      throw new TypeError('Array items can only be integers or arrays of integers.')
    }

    flat.push(entry)
  }

  return flat
}

// Make some assertions
assert.deepEqual(flatten([[1, 2, [3]], 4]), [1, 2, 3, 4])
assert.throws(flatten.bind(null, [1, 2, { foo: 'bar' }]), expectedError)
assert.throws(flatten.bind(null, { foo: 'bar' }), expectedError)

// Allow running from the CLI where arguments are passed as JSON strings
if (process.argv[2]) {
  try {
    console.log(flatten(JSON.parse(process.argv[2])))
  } catch (error) {
    console.error(`Error: ${error.message}`)
    process.exit(1)
  }
}

// Allow requiring as a module
module.exports = flatten

node <path-to-script> "[[1, 2, [3]], 4]"

or

const flatten = require('path-to-module')

flatten([[1, 2, [3]], 4])