To suppress the default Microsoft® Internet Explorer error message for the window event, set the returnValue property of the event object to true or simply return true in Microsoft JScript®.
The onerror event fires for run-time errors, but not for compilation errors. In addition, error dialog boxes raised by script debuggers are not suppressed by returning true. To turn off script debuggers, disable script debugging in Internet Explorer by choosing Internet Options from the Tools menu. Click the Advanced tab and select the appropriate check box(es).