flatDeep.js

/**
 * Function for converting arbitrarily nested arrays into a single, flat array
 * @param {Array} arr
 * @returns {Array}
 */
function flatDeep (arr) {
  if (!Array.isArray(arr)) throw new Error('Expected type `Array`')
  return arr.reduce((acc, el) => acc.concat(Array.isArray(el) ? flatDeep(el) : el), [])
}

// Tests
const assert = require('assert')
assert.throws(() => flatDeep('foo'), Error)
assert.deepEqual(flatDeep([[1,2,[3]],4]), [1, 2, 3, 4])
assert.deepEqual(flatDeep([1,,3,[4]]), [1,3,4])
assert.deepEqual(flatDeep([1, ['foo', true]]), [1,'foo',true])
assert.deepEqual(flatDeep([[1], [{ foo: 'bar' }]]), [1,{foo: 'bar'}])