Flatten an Array in Javascript

flatten.js

exports.first = function(l) {
  return l[0]
};

exports.rest = function(l) {
  return l.reduce(function(accumulator, item, index) {
    if(index > 0) {
      accumulator.push(item);
    }
    return accumulator;
  }, []);
};

exports.flatten = function(l, accumulator){
  if(l.length == 0) {
    return accumulator;
  } else {
    var head = exports.first(l);
    if(Array.isArray(head)) {
      return exports.flatten(head, accumulator);
    } else {
      accumulator.push(head);
      return exports.flatten(exports.rest(l), accumulator);
    }
  }
};

Use in node.js:

> var x = require('./flatten.js');
> x.flatten([1, 2], [])
[ 1, 2 ]
> x.flatten([1, [2,3]], [])
[ 1, 2, 3 ]
> x.flatten([1, [2,3, [4, 5, 6]]], [])
[ 1, 2, 3, 4, 5, 6 ]
> x.flatten([1, [2,3, [4, 5, [6]]]], [])
[ 1, 2, 3, 4, 5, 6 ]