var arr = ['a', 'b', 'c', 'c', 'd','e', 'e', 'e', 'e', 'e', 'f', 'e', 'f', 'e', 'f', 'a', 'a', 'a', 'f', 'f', 'f'];
var last = ”;
var second_last = ”;
var opened = false;
var res = “”;
for(var i in arr) {
if(arr[i] == last && last == second_last && opened == false) {
res += “”;
opened = true;
}
if(opened && arr[i] != last) {
res += “”;
opened = false;
}

second_last = last;
last = arr[i];

res += arr[i];
}
if(opened) res += “”;
alert(res);