javascript - confirm use of jQuery.ajaxSettings.xhr - Stack Overflow

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var _orgAjax = jQuery.ajaxSettings.xhr;
jQuery.ajaxSettings.xhr = function () {
    var xhr = _orgAjax();
    DoSomeLogging();
    return xhr;
};

I've inherited the code above. Can someone confirm my understanding that this will override any $.ajax(...) call anywhere else on the page and execute DoSomeLogging() each time?

I'm hunting a different bug but stumbled upon this and wanted to be sure I understand what this piece of legacy code does?

I'm not convinced this is good practice/use of the xhr function anyway but interested to hear your opinions. We're using Jquery 1.7.1.

Lastly, out of curiosity, what if _orgAjax() failed to return if an (async: false) call, would the logging method even get reached?

Thanks.

var _orgAjax = jQuery.ajaxSettings.xhr;
jQuery.ajaxSettings.xhr = function () {
    var xhr = _orgAjax();
    DoSomeLogging();
    return xhr;
};

I've inherited the code above. Can someone confirm my understanding that this will override any $.ajax(...) call anywhere else on the page and execute DoSomeLogging() each time?

I'm hunting a different bug but stumbled upon this and wanted to be sure I understand what this piece of legacy code does?

I'm not convinced this is good practice/use of the xhr function anyway but interested to hear your opinions. We're using Jquery 1.7.1.

Lastly, out of curiosity, what if _orgAjax() failed to return if an (async: false) call, would the logging method even get reached?

Thanks.

• javascript
• jquery
• ajax
• xmlhttprequest

Share Improve this question Follow edited Aug 12, 2012 at 22:54 Fabrício Matté 70.2k2727 gold badges134134 silver badges167167 bronze badges asked Aug 12, 2012 at 22:50 BarryFantaBarryFanta 6911 gold badge11 silver badge77 bronze badges 3

• Have you tried to test this by yourself (or is it for yourself - not sure)? Create a new page, insert the above code, make some $.ajax() requests. Is DoSomeLogging invoked? – Šime Vidas Commented Aug 12, 2012 at 22:59
• How can _origAjax fail to return? You mean, if an error is thrown within it? In that case, obviously, the entire JavaScript program would terminate (stop executing). – Šime Vidas Commented Aug 12, 2012 at 23:08
• The request is not made at this point so it doesn't matter if it's synchronous or not. – Esailija Commented Aug 12, 2012 at 23:11

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2 Answers 2

Sorted by: Reset to default 4

Can someone confirm my understanding that this will override any $.ajax(...) call anywhere else on the page and execute DoSomeLogging() each time?

Yes. This will define the global default behaviour unless overwritten in a local ajax implementation.

The more documented way of defining global settings is the use of ajaxSetup().

However, you are able to set default global behaviour using ajaxSettings as well, though this particular way is not documented anywhere directly in the jQuery documentation.

See DEMO - With Success

Lastly, out of curiosity, what if _orgAjax() failed to return if an (async: false) call, would the logging method even get reached?

The settings are configuring the beforeSend so the logging method will always be called regardless of the async setting as pointed out correctly by Esailija.

See DEMO - with async: true, logging is called before return
See DEMO - with async: false, logging is also called before return

However please note though, according to the ajax documentation:

As of jQuery 1.8, the use of async: false with jqXHR ($.Deferred) is
deprecated; you must use the plete/success/error callbacks.

I know you are using 1.7.1 but if something bees deprecated there usually is a good reason and I don't see any harm in preparing for that.

To configure ajax with logging on plete by default:

See DEMO - Configuring default logging for plete for document. I know that would make your logging being executed after and not before the send but that seems to be the way 1.8 prefers it I think.

Hm, you could:

$.ajaxSetup({
    beforeSend: function () {
        doSomeLogging();   
    }
});

That's certainly simpler than having to redefine $.ajaxSettings.xhr.

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