Merges (flattens) all inner arrays into one-level depth array

flatten-es6.js

/**
 * Merges all inner arrays into one-level depth array.
 *
 * @param  {Array} array: the array to be flattened
 * @return {Array}
 */
const flatten = array => array.reduce(
  (flattened, cv) => flattened.concat(Array.isArray(cv) ? flatten(cv) : cv),
  [] // initial value of flattened array
);

flatten.js

/**
 * Merges all inner arrays into one-level depth array.
 *
 * @param  {Array} array: the array to be flattened
 * @return {Array}
 */
function flatten(array) {
  return array.reduce(function (flattened, cv) {
    return flattened.concat(Array.isArray(cv) ? flatten(cv) : cv);
  }, []); // initial value of flattened array
}

usage.js

var array = [[1], [2], [3, 4], [['a', 'b'], [['c'], 'd']], 9];

// merges all inner arrays into one-level array
var flat = flatten(array);
console.log(flat);