flattenArray.js

import { isArray } from './utils';

export default function flattenArray(arr) {
  
  // Use Array.reduce to get our result
  let flattened = arr.reduce(function(current, item) {
    // Store a reference to the current item
    let temp = item;
    
    // Check if the item is an array
    if (isArray(item)) {
      // If it is an array, then apply the function again (recursively) and store the result in temp
      temp = flattenArray(item);
    }
    
    // concat temp with the current result
    return current.concat(temp);
  }, []);

  return flattened;
}

test.js

import test from 'ava';
import { isArray } from './utils';
import flattenArray from './flattenArray';

test(t => {
  let testPass = [[1,2,[3, [5, 6]]],4];
  let testFail = {};
  
  t.true(isArray(testPass));
  t.false(isArray(testFail));
});

test(t => {
  let test = [[1,2,[3, [5, 6]]],4];
  let expected = [ 1, 2, 3, 5, 6, 4 ];
  let result = flattenArray(test);
  
  t.same(result, expected);
});

utils.js

// Check is something is an Array, use Array.isArray if it's supported, otherwise use instanceOf
export function isArray(arr) {
  return Array.isArray ? Array.isArray(arr) : arr instanceof Array;
}