pop() mutates the underlying list: it removes the last item (and returns it).
It seems you're looking for slice:
"www.mycompany.com/sites/demo/t1".split('/').slice(0, -1).join('/')
// -> gives: "www.mycompany.com/sites/demo"
.slice(0, -1) gives the elements of the list from the start until the end minus one item.
However, this is a very poor solution to drop the last path element from a URL. In particular, splitting a string to an array, drop the last element and join the string again is inefficient. It would be better to use lastIndexOf to find the position of the last /, and then substr to extract the part of the string before the last /.
var s = "www.mycompany.com/sites/demo/t1";
s.substr(0, s.lastIndexOf('/'));