Find the common words in two strings and return an alphabetically sorted string of common words

common-words.js

function commonWords (first, second) {
  var a = first.split(',')
  var b = second.split(',')
  var c = []
  for (var i = 0; i < a.length; i++) {
    for (var j = 0; j < b.length; j++) {
      if (a[i] === b[j] && c.indexOf(a[i]) !== null) {
        c.push(a[i])
      }
    }
  }
  return c.sort().toString()
}