Common Mistakes, Debugging, and Error Handling - Beginning JavaScript - page 561

Let's first look at the checkUsername() function. It has been rewritten to use the try...catch and
throw statements for validating the username <input/> element, and the majority of this function's
code resides within the try block:
try {
if (!userValue) {
throw {
message: "Please enter a user name to check!"
};
}
var parms = {
username: userValue
};
$.getJSON("ch14_formvalidator.php", parms).done(handleResponse);
}
Before you make the Ajax request, you first ensure the user provided a value to the username field. If userValue
is blank, you throw a new object detailing the cause of the exception with its message property. This causes
the JavaScript engine to stop executing code in this try block and starts executing the catch block:
catch (ex) {
alert(ex.message);
}
Here, you simply alert the exception's message property, displaying the “ Please enter a user name
to check! ” message to the user.
Naturally, the changes made to the checkEmail() function are almost identical to checkUsername() :
try {
if (!emailValue) {
throw {
message: "Please enter an email address to check!"
};
}
var parms = {
email: emailValue
};
$.getJSON("ch14_formvalidator.php", parms).done(handleResponse);
} catch (ex) {
alert(ex.message);
}
Once again, the majority of the function code resides within a try code block. If the e‐mail field
validation fails, you throw an object containing the exception message and display that message in an
alert box—the result of executing the code in the catch block.