Array to string using array methods

This way may seem to be uselss becuase you are using anonymous function to acomplish something that you can do it with join(); But if you need to make something to the strings while you are converting the Array to String, this can be useful.

var  arr = ['a', 'á', 'b', 'c']

function upper_lower (a, b, i) {
  //...do something here
  b = i & 1 ? b.toUpperCase() : b.toLowerCase();
  return a + ',' + b
}
arr = arr.reduce(upper_lower); // "a,Á,b,C"