Can a script cause memory leaks? Or does JavaScript have a build in garbage collector like Java?

More specifically, I have a global array variable which I often need to overwrite with

arrGlobalArray = new Array();
arrGlobalArray[0] = "new item";

what happens when I renew the array with

arrGlobalArray = new Array();
arrGlobalArray[0] = "different item";

Browser is IE6 if that helps.