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Caching, jQuery Ajax and Other IE Fun

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In the last couple of days I had quite a fun time debugging problems of our JavaScript SPAs on oldIE (thanks God we got the Browserstack licenses). Anyway, I encountered some quite interesting behavior of IE with jQuery Ajax calls and cached responses.

“HTTP/1.1 304 Not Modified”: No Way! You Won’t Catch Me

To have this experiment work, ensure you have your browser caches active.

Take the following ajax request to a static JSON file

$.ajax({
        url: '/data.json',
        type: 'GET',
        dataType: 'JSON',
        success: function(data, textStatus, jqXHR){
            console.log('Data received: ' + data);
        }
    });

The first request probably returns with a Http/1.1 200 OK while any subsequent (if caching is activated on the server) will return with a Http/1.1 304 Not Modified. You nicely see this in Chrome’s Network panel

Now, assume you’d like to perform some operation just on Http/1.1 200 and not on 304 requests as well. Easy enough, right? You simply check against the jqXHR.status object:

...
success: function(data, textStatus, jqXHR){
    if(jqXHR.status === 200){
        console.log('Got a good request');
    }
}

Interestingly though, for the above request I’d get

Got a good request
Got a good request
Got a good request

…printed on my console and indeed, when checking against the jqXHR request, the jqXHR.status returns 200 for Http/1.1 200 as well as for Http/1.1 304. Why this?? This is by design of the XMLHttpRequest object.

For 304 Not Modified responses that are a result of a user agent generated conditional request the user agent must act as if the server gave a 200 OK response with the appropriate content. […] w3c.org [RFC2616]

Wait..there is a way!

Actually there is a way to also get the 304 status codes in your jQuery ajax callback. Again:

The user agent must allow setRequestHeader() to override automatic cache validation by setting request headers (e.g., If-None-Match, If-Modified-Since), in which case 304 Not Modified responses must be passed through. w3c.org [RFC2616]

But note, you don’t have access to the browser’s If-Modified-Since or E-Tag information. jQuery however keeps track about it and allows you to use the ifModified flag on the ajax options:

$.ajax({
    url: '/data.json',
    type: 'GET',
    dataType: 'JSON',
    ifModified:true,
    success: function(data, textStatus, jqXHR){
        if(jqXHR.status === 200){
            console.log('Got a good request');
        } else {
            console.log('Hey, a 304 request');
        }

        console.log('Data: ' + data);
    }
});

Executing this now again, where the first request returns a 200 and the subsequent ones a 300 response status gives me the following on the log:

Got a good request
Data: <snip>JSON string</snip>
Hey, a 304 request
Data: undefined
Hey, a 304 request
Data: undefined

Hell, wtf?? Yep, again this is by design:

[…] in which case 304 Not Modified responses must be passed through […]

..and indeed, a 304 response does not carry any data.

Ok, but Why Should This Be An Issue?

Normally it won’t, just don’t put the ifModified flag and you’re fine. In my specific scenario however, I had to analyze for changes in a specific response header, so I used the ajaxComplete(..) function like

var currentHeaderVal = undefined;

$(document).ajaxComplete(function(e, xhr, settings){
    var newHeaderVal = xhr.getResponseHeader('MY-HEADER');
    if(currentHeaderVal !== newHeaderVal){
        //do something fancy
    }
});

On the server-side some mechanism injected the MY-HEADER value. The idea is simple and it also works (on Chrome, Firefox and any other decent browser) but, surprise, surprise, on IE it doesn’t. When using xhr.getResponseHeader('MY-HEADER'), IE returns the headers from the previously cached request which obviously are not necessarily valid any more.

As such, the idea was to check the xhr.status in order to just read the headers from 200 type requests, like

var newHeaderVal = xhr.getResponseHeader('MY-HEADER');
if(xhr && xhr.status === 200){
    if(currentHeaderVal !== newHeaderVal){
        //do something fancy
    }
}

…but as we know now, that won’t work.

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Published at DZone with permission of Juri Strumpflohner, DZone MVB. See the original article here.

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