Taken from http://stackoverflow.com/questions/1916218/find-the-longest-common-starting-substring-in-a-set-of-strings

longest-common-starting-substring-in-array.js

function sharedStart(array) {
  var A= array.concat().sort(), 
  a1= A[0], a2= A[A.length-1], L= a1.length, i= 0;
  while(i<L && a1.charAt(i)=== a2.charAt(i)) i++;
  return a1.substring(0, i);
}