Using $.ajaxPrefilter() To Configure AJAX Requests In jQuery 1.5

Posted February 18, 2011 at 6:29 PM

Last week, I started to explore Deferred objects and the AJAX updates made to jQuery 1.5 byJulian
Aubourg. To kick off the exploration, I created a function that
would essentially proxy the XHR request object, normalizing the core
response in order to account for web
services that use meaningful HTTP status codes in order to define
request errors. In the comments to that post, Julian Aubourg himself
pointed out that I could use the new $.ajaxPrefilter() method as a means
to implement the same functionality in a much
more flexible way. To help wrap my head around his comments, I decided
to refactor my previous post using a combination of $.ajaxSetup() and
$.ajaxPrefilter().

As a quick recap, jQuery only parses "success" responses returned from
the server. That means that requests with 40x and 50x HTTP status codes
do not get parsed. The problem with this is that web services often
return status codes like 400 and 401 in order
to provide a better context for the response data. In order to make
sure that jQuery parses the JSON data associated with these non-200
responses, we need to proxy the AJAX request and augment the routing
logic.

In my previous post, I created this AJAX proxy by manually wrapping the
core jqXHR object as part of my AJAX request. As Julian pointed out,
however, it would be much better if I could leave this kind of object
decoration up to a configuration option. By using
his approach, not only would I factor out the burden of creating the
proxy object, I would also allow the request normalization to be done
either on a global or one-off basis.

To see what I'm talking about, take a look at this demo code. As you
read through it, notice that I am using $.ajaxSetup() to apply the
normalization and then $.ajaxPrefilter() to implement it.

NOTE: I am not going to reproduce the ColdFusion
API code since it is not really relevant to this exploration. If you
like, you can alway refer
to my previous post.

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<!DOCTYPE html>
<html>
<head>
<title>Using $.ajaxPrefilter() With AJAX In jQuery 1.5</title>
<script type="text/javascript" src="../jquery-1.5.js"></script>
</head>
<body>
<h1>
Using $.ajaxPrefilter() With AJAX In jQuery 1.5
</h1>
<form>
<h2>
Enter Data
</h2>
<p class="message" style="display: none ;">
<!--
This is where the confirmation message will go
on the AJAX request completion.
-->
</p>
<p>
Username:
<input type="text" name="username" size="20" />
</p>
<p>
Name:
<input type="text" name="name" size="20" />
</p>
<p>
Age:
<input type="text" name="age" size="5" />
</p>
<p>
<input type="submit" value="Save Contact" />
</p>
</form>
<script type="text/javascript">
// Store DOM references.
var form = $( "form" );
var message = $( "p.message" );
var username = form.find( "input[ name = 'username' ]" );
var contactName = form.find( "input[ name = 'name' ]" );
var contactAge = form.find( "input[ name = 'age' ]" );
// This page is part of an application that calls a web
// service that returns response values with the most
// appropriate status codes. We want those status codes
// to trigger done/fail callbacks with parsed values.
$.ajaxSetup(
{
normalizeForStatusCodes: true
}
);
// Bind to the form submission error to handle it via AJAX
// rather than through the standard HTTP request.
form.submit(
function( event ){
// Prevent the default browser behavior.
event.preventDefault();
// Try to save the contact to the server. The
// saveContact() method returnes a promise object
// which will come back with a result eventually.
// Depending on how it resolves, either the done()
// or fail() event handlers will be invoked.
//
// NOTE: This return object can be chained; but for
// clarity reasons, I am leaving these as one-offs.
var saveAction = saveContact(
username.val(),
contactName.val(),
contactAge.val()
);
// Hook into the "success" outcome.
saveAction.done(
function( response ){
// Output success message.
message.text(
"Contact " + response.data + " saved!"
);
// Show the message.
message.show();
}
);
// Hook into the "fail" outcome.
saveAction.fail(
function( response ){
// Output fail message.
message.html(
"Please review the following<br />-- " +
response.errors.join( "<br />-- " )
);
// Show the message.
message.show();
}
);
}
);
// I save the contact data.
function saveContact( username, name, age ){
// Initiate the AJAX request. This will return an
// AJAX promise object that maps (mostly) to the
// standard done/fail promise interface.
var request = $.ajax({
type: "post",
url: "./api.cfm",
data: {
username: username,
name: name,
age: age
}
});
// Return the jqXHR promise object.
return( request );
}
// -------------------------------------------------- //
// -------------------------------------------------- //
// -------------------------------------------------- //
// -------------------------------------------------- //
// -------------------------------------------------- //
// -------------------------------------------------- //
// Here, we are providing a way to normalize AJAX responses
// to web services make proper use of status codes when
// returning request values. This will look for a
// "normalizeForStatusCodes" option before altering the
// jqXHR object.
$.ajaxPrefilter(
function( options, localOptions, jqXHR ){
// Check to see if this request is going to require
// a normalization based on status codes.
if (options.normalizeForStatusCodes){
// The user wants the response status codes to
// be handled as part of the routing; augment the
// jqXHR object to parse "fail" responses.
normalizeAJAXRequestForStatusCodes( jqXHR );
}
}
);
// I take the AJAX request and return a new deferred object
// that is able to normalize the response from the server so
// that all of the done/fail handlers can treat the incoming
// data in a standardized, unifor manner.
function normalizeAJAXRequestForStatusCodes( jqXHR ){
// Create an object to hold our normalized deferred.
// Since AJAX errors don't get parsed, we need to
// create a proxy that will handle that for us.
var normalizedRequest = $.Deferred();
// Bind the done/fail aspects of the original AJAX
// request. We can use these hooks to resolve our
// normalized AJAX request.
jqXHR.then(
// SUCCESS hook. ------ //
// Simply pass this onto the normalized
// response object (with a success-based resolve).
normalizedRequest.resolve,
// FAIL hook. -------- //
function( jqXHR ){
// Check to see what the status code of the
// response was. A 500 response will represent
// an unexpected error. Anything else is simply
// a non-20x error that needs to be manually
// parsed.
if (jqXHR.status == 500){
// Normalize the fail() response.
normalizedRequest.rejectWith(
this,
[
{
success: false,
data: "",
errors: [ "Unexpected error." ],
statusCode: jqXHR.statusCode()
},
"error",
jqXHR
]
);
} else {
// Normalize the non-500 "failures." These
// are actually valid responses that require
// actions on the part of the user.
normalizedRequest.rejectWith(
this,
[
$.parseJSON( jqXHR.responseText ),
"success",
jqXHR
]
);
}
}
);
// We can't actually return anything meaningful from this
// function; but, we can augment the incoming jqXHR
// object. Right now, the incoming jqXHR promise methods
// reference their original settings; however, we can
// copy the locally-created deferred object methods into
// the existing jqXHR. This will keep the jqXHR objec the
// same reference -- but, it will essentially change all
// the meaningful bindingds.
jqXHR = normalizedRequest.promise( jqXHR );
// At this point, the promise-based methods of the jqXHR
// are actually the locally-declared ones. Now, we just
// have to point the sucecss and error methods (AJAX
// related) to the done and fail methods (promise
// related).
jqXHR.success = jqXHR.done;
jqXHR.error = jqXHR.fail;
// NOTE: No need to return anything since the jqXHR
// object is being passed by reference.
}
</script>
</body>
</html>

At the top of the code, I am using $.ajaxSetup() to indicate that all
AJAX requests will be made to a web service that uses targeted HTTP
status codes in order to help define its response.

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$.ajaxSetup(
{
normalizeForStatusCodes: true
}
);

Really, all this is doing is helping to define a base hash of AJAX
configuration options that will be used by AJAX requests on the current
page. This can be overridden by each AJAX request - $.ajax(); but for
our purposes, we'll stick with the global configuration.

And, once we have the global configuration in place, we use the
$.ajaxPrefilter() method to augment the jQuery XHR object (jqXHR) based
on the configuration options.

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$.ajaxPrefilter(
function( options, localOptions, jqXHR ){
if (options.normalizeForStatusCodes){
normalizeAJAXRequestForStatusCodes( jqXHR );
}
}
);

The function reference passed to the $.ajaxPrefilter() method will be
executed for every single AJAX request. In this case, we are looking at
the merged options hash (NOTE: localOptions are the options defined
directly by the $.ajax() method) and, if the "normalizeForStatusCodes"
option is true, we are augmenting the jqXHR object.

The augmentation of the jqXHR object is being encapsulated within the
function, normalizeAJAXRequestForStatusCodes(). I am doing this mostly
because it helps me think about the problem in isolated,
compartmentalized steps. Of course, we aren't just moving code
around - we're changing the way it works; and, when we use
$.ajaxPrefilter(), we can't return a proxy object in the same way that
we were doing before. Rather, we need to directly manipulate the
outgoing jqXHR object.

As per Julian's lead, I am doing this by calling the promise() method on my deferred-bridge and passing-in the jqXHR object:

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jqXHR = normalizedRequest.promise( jqXHR );
jqXHR.success = jqXHR.done;
jqXHR.error = jqXHR.fail;

The name, "promise," is kind of misleading here - we're not really
creating a new object; when you pass an object (as an argument) into the
promise() method, what you're actually doing is copying the method
references from one object to the other. So, in this
case, we are copying all of the promise-based methods - then, done,
fail, isResolved, isRejected, promise - from the normalizedRequest
Deferred object into the jqXHR object.

This is actually super interesting! This works because the object
reference never changes - only its properties do. And, since the
properties consist
of methods that are lexically bound, you're essentially creating an
in-place proxy object, leaving the same outer shell intact. There's
something deeply satisfying about that.

I'm still wrapping my head around all of the new jQuery 1.5
functionality; but, a huge thanks to Julian for point this stuff out to
me. There are, of course, other ways to configure outgoing AJAX requests
in jQuery such as the beforeSend callback; but, this
combination of $.ajaxSetup() and $.ajaxPrefilter() just feels very
clean.

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