array-flatten.js

function flatten(arr) {
  // The reduce() method reduces the array to a single value
  // @see http://www.w3schools.com/jsref/jsref_reduce.asp
  return arr.reduce(function (flat, toFlatten) {
    // The concat() method is used to merge two or more arrays.
    // This method does not change the existing arrays, but instead returns a new array.
    // @see https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/concat
    return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
  }, []);
}