1  module.exports = function (xs, fn) {
 2      var res = [];
 3      for (var i = 0; i < xs.length; i++) {
 4          var x = fn(xs[i], i);
 5          if (isArray(x)) res.push.apply(res, x);
 6          else res.push(x);
 7      }
 8      return res;
 9  };
10  
11  var isArray = Array.isArray || function (xs) {
12      return Object.prototype.toString.call(xs) === '[object Array]';
13  };