Implement program flow - Programming in HTML5 with JavaScript and CSS3 - page 150

With errors like this, users will likely stop coming to the website or using the application.
To prevent such issues, you need to wrap the code in a try block:

try{
window.dosomeunsupportedmethod();
} catch (e) {
alert("Browser does not support the desired functionality.");
}

By using the try…catch block, you can handle the error condition gracefully. The users see
a standard alert and the webpage continues to run as usual. This example code results in the
message shown in Figure 2-4.

FIGURE 2-4 A clean message box to show errors

The try…catch block is divided into two parts. The first part, the try portion, says, “Try to
do this work.” If anything goes wrong when trying to do the work, the catch block receives
an exception object with information about the error. Any code inside the try portion of the
try…catch block is protected against encountering an unhandled error.
The catch block is where the error can be handled as appropriate for the application. The
catch block receives a parameter that is an exception object. Table 2-8 shows the properties
for the exception object.

TABLE 2-8 Properties available on the exception object

Property Description

A textual description of the error that occurred message

A numeric error code

number

The name of the exception object

name

You can use the information provided in the exception object to decide what to do in
terms of overall program flow. For example, if the program needs access to a resource that
it can't have and an exception is thrown, the program can fall back to a different process
to achieve the desired functionality or simply tell the user that something needs to be
changed—for example, if cookies or another HTML5 API are required for the site to work.
Other ways to check for this type of thing are demonstrated shortly.