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We already introduced Visual Studio’s Exceptions dialog in Chapter 2; refer back to Figure 2-16. For
each type of JavaScript exception, this dialog supplies two checkboxes labeled Thrown and
User-unhandled. Checking Thrown will display a dialog box in the debugger (Figure 3-5) whenever an
exception is thrown, regardless of whether it’s handled and before reaching any of your error handlers.
If you have error handlers, you can safely click the Continue button in the dialog, and you’ll eventually
see the exception surface in those error handlers. (Otherwise the app will terminate.) If you click Break
instead, you can find the exception details in the debugger’s Locals pane, as shown in Figure 3-6.
FIGURE 3-5 Visual Studio’s exception dialog. As the dialog indicates, it’s safe to press Continue if you have an error
handler in the app; otherwise the app will terminate. Note that the checkbox in this dialog is a shortcut to toggle the
Thrown checkbox for this exception type in the Exceptions dialog.
FIGURE 3-6 Information in Visual Studio’s Locals pane when you Break on an exception.
The User-unhandled option (enabled for all exceptions by default) will display a similar dialog
whenever an exception is thrown to the event loop, indicating that it wasn’t handled by an appprovided
error function (“user” code from the system’s perspective).
You typically turn on Thrown only for those exceptions you care about; turning them all on can make
it very difficult to step through your app! Still, you can try it as a test, and then leave checks only for
those exceptions you expect to catch. Do leave User-unhandled checked for everything else; in fact,
unless you have a specific reason not to, make sure that User-unhandled is checked next to JavaScript
Runtime Exceptions here because this will include those exceptions not otherwise listed. This way you
can catch (and fix) any exceptions that might abruptly terminate the app, which is something your
customers should never experience.
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