I have a document library and I would like to update the metadata(columns) of a document using JSOM. I thought the following would work but it works just on custom list.

```
 var ctx = new SP.ClientContext.get_current();
                            var customLibrary = ctx.get_web().get_lists().getByTitle('Shared Documents');
                            var listItem = customList.getItemById(itemID);

                            /*Set the value and update*/
                            listItem.set_item('Project_0020_Number', prjNum);
                            listItem.update();

                            ctx.executeQueryAsync(
                            function(){ 
                                                                            /*Need to change this to show on the page*/
                                                                            alert('Updated'); 
                                            }, 

                                            function(sender, args){ alert('Error: ' + args.get_message()); });
                                            }

                                            function onQueryFailed(sender, args) {
                                                            alert('Request failed. '+args.get_message() + '\n' + args.get_stackTrace());

```