Error trapping with JavaScript only works for newer generation browsers. However, older browser generally ignore window.onerror code, so it is worth putting in. There are two ways to do this:

Handling Errors via the window.onerror DHTML Event

function reportError(msg,url,line) {
  // do what you want with the message (msg), url (url) 
  // and line number (line) here
  return true;

window.onerror = reportError;

Handling Errors via ECMAScript 2.0 Exception Handling

  try {
    // code that may cause an error
  catch (exception) {
    alert(err.number + ' ' + err.description);
  finally {
     // code that is always executed