Use DefaultDisplayFormUrl at the time of loading the list in Javacript client object model. Example:
var clientContext = new SP.ClientContext.get_current();
var list = clientContext.get_web().get_lists().getByTitle(listTitle);
clientContext.load(list, 'DefaultDisplayFormUrl');
Then while enumerating the listitemcollection use get_defaultDisplayFormUrl() to create the Url. Example:
while (listItemEnumerator.moveNext()) {
var listItem = listItemEnumerator.get_current();
var listItemId = listItem.get_item('ID');
var href = list.get_defaultDisplayFormUrl() + "?ID=" + listItemId;
}