When a runtime error occurs, web browsers show specified error messages and may offer the possibility to examine this error with an debugger.

By applying an function pointer to window.onerror, it is possible to handle this error by the specified function and optionally disable the browser’s error handling by returning true.

window.onerror = handleError;

The function takes three parameters:

• message the error message
• location filename of the script containing the erroneous code
• number the error’s code number

function handleError(message, url, code){
  //..
  return true;
}

Unfortunately, every browser behaves a bit different:

• Internet Explorer handles errors to the given function
• Firefox handles errors and can handle uncaught exceptions to the function
• Safari only shows the error in the JavaScript Console
• Opera might show the error in the error console

Thrown exceptions can be handled to a specified function to perform further examinations.

try
{
  var foo = '';
  foo.bar();
}
catch (exception)
{
  handleException(exception)
}

The exception object contains the following properties:

• name the name of the exception (e.g. TypeError, SyntaxError)
• message a text describing the error

In some browsers the object has more properties:

• Internet Explorer:
  • number error code
  • description description of the error (mostly identical to message)
• Firefox:
  • stack call stack
  • fileName name of the script containing the erroneous code
  • lineNumber line which contains the error