When communicating with a web server, problems can arise.

You may use AJAX to request a file that's not on the server, or

the server might encounter a technical error

and be unable to process your request.

jQuery's AJAX functions fail silently.

Meaning, there won't be any big error when a request fails, but this also means

the success callback, which processes the response

and updates the webpage, won't do anything either.

In many cases, that might be okay, but sometimes you

want to let your users know when there's a problem.

For example, say you're building an AJAX-driven chat application.

Users can type a message into a form field,

hit Submit, and the message is sent to the server.

On the other end of the conversation,

another user also has the chat application.

Using AJAX requests, any new messages from the server are

delivered and the webpage is updated with the new message.

But what if there's a problem?

The server isn't working and can't receive a

new chat message, or it can't send that message.

In this case, it's a good idea to notify the users of the problem.

Otherwise, they'll think the other person in the chat is ignoring them.

For each of jQuery's AJAX methods, like the get, post, load

and AJAX methods, there is an additional method to handle any problems.

Its purpose is to run a function when an AJAX request fails.

Let's see how this works.

Say you're using jQuery's $.get method.

This method makes a basic get request to the server and receives a response.

For example.

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This code loads the contents of the file missing.html into an

HTML tag on the page that has the ID of mydiv.

If the server has a problem or that file is missing,

you can add a function that runs only when the request fails.

jQuery's fail method can be tagged directly to the end

of any of jQuery's AJAX methods, except the .load method.

jQuery supports a process known as chaining, where some methods can

be tacked on directly at the end of the last method.

In this case, we can simply add .fail to the end of .get.

The fail method takes a single argument, a

callback function that's run when the request fails.

jQuery passes several arguments to this callback, but the only one that's

really useful is the first argument, which is a jQuery XHR object.

The jQuery XHR object is an enhanced version of a regular XML HTTP

request object, which you learned about in the second stage of this course.

jQuery provides a bunch of extra

properties that are useful for AJAX programming.

The jQuery XHR object contains some important information about the error.

For example, you can retrieve the HTTP error code.

Let's see what happens when we wrap this in an alert function,.

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And preview the page.

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404, that's a file not found error code.

The server also sends text, like not found, or internal

server error, and that's stored in a property named statustext.

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You can use that information to explain the error to the user.

For example, if you wanted to print out the error

message when the get method fails, you could do this.

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If you simply want to print something to the

page when there's an error, there's an even easier way.

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One thing to keep in mind about jQuery's

fail method, it doesn't work for $.load method, and

it doesn't work for remote AJAX requests that use

JSONP that is request to a different web server.

So if you're using AJAX to download tweets from Twitter

or photos from Flickr, the fail method won't ever run.

For those cases, you need to check the response in

the normal success callback to see how the server responded.

Errors do happen, so it's a good idea to plan for them.

When the AJAX functionality isn't critical to

the webpage, for example, you're using it just

to display the current weather, it's fine to

just let a bad AJAX request fail silently.

But in cases where the AJAX request and response

are important to the usability of the page, it's

a good idea to use the fail method to

let your site visitors know when there's a problem.