Who can help to explain this JavaScript algorithm [].filter.call()

Question

I have task to receive unique element in order from string as parameter. I do not understand how this function uniqueElements returns [\'A\',\'B\',\'C\',\'B\']

Answer 1

var word = "AAAABBBBCCBB";
var unique = word.split('').filter(function(item, i, ar) {
  return ar.indexOf(item) === i;
}).join('');