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

I would do this O(n) time by utilizing Array.prototype.reduce() as follows;

var str = "AAAABBBBCCBB",
uniques = Array.prototype.reduce.call(str, (p,c,i) => i-1 ? p[p.length-1] !== c ? (p.push(c),p)
                                                                                : p
                                                          : [c]);
console.log(uniques);