how to overwrite the success function via JQuery ajaxSend event

Question

I am trying to overwrite the success function upon ajaxsend event but it doesnt work here is the code:

    $(document).ajaxSend(function(event,xhr,options){
        console.log('ajaxSend');
        var tempSuccess = options.success;
        options.success = function(data, textStatus, jqXHR){
            console.log('start');
            tempSuccess(data, textStatus, jqXHR);
            console.log('end');
        }; xhr.success = options.success;});

upon AJAX I do see 'ajax' in the console, but upon success I can't see the start and the end debug msges..

What do I do wrong?

Answer 1

What you're trying to accomplish can't be done with ajaxSend. The problem is that ajaxSend apparently works with a copy of the original xhr and options objects, so the modifications won't have any effect. You can easily test this with the following code:

$(document).ajaxSend(function(event, xhr, options){
    delete options.success;
    console.log(options.success);   // undefined
});
$.ajax({
    url: "test.html",
    success: function() { console.log("this will be printed nevertheless"); }
});

So you can't use ajaxSend to overwrite the success callbacks. Instead, you will have to "hack" jQuery's AJAX function:

// closure to prevent global access to this stuff
(function(){
    // creates a new callback function that also executes the original callback
    var SuccessCallback = function(origCallback){
        return function(data, textStatus, jqXHR) {
            console.log("start");
            if (typeof origCallback === "function") {
                origCallback(data, textStatus, jqXHR);
            }
            console.log("end");
        };
    };

    // store the original AJAX function in a variable before overwriting it
    var jqAjax = $.ajax;
    $.ajax = function(settings){
        // override the callback function, then execute the original AJAX function
        settings.success = new SuccessCallback(settings.success);
        jqAjax(settings);
    };
})();

Now you can simply use $.ajax as usual:

$.ajax({
    url: "test.html",
    success: function() {
        console.log("will be printed between 'start' and 'end'");
    }
});

As far as I know, any of jQuery's AJAX functions (such as $.get() or .load()) internally use $.ajax, so this should work with every AJAX request done via jQuery (I haven't tested this though...).

---

Something like that should also work with "pure" JavaScript by hacking the XMLHttpRequest.prototype. Note that the following won't work in IE, which uses ActiveXObject instead of XMLHttpRequest.

(function(){
    // overwrite the "send" method, but keep the original implementation in a variable
    var origSend = XMLHttpRequest.prototype.send;
    XMLHttpRequest.prototype.send = function(data){
        // check if onreadystatechange property is set (which is used for callbacks)
        if (typeof this.onreadystatechange === "function") {
            // overwrite callback function
            var origOnreadystatechange = this.onreadystatechange;
            this.onreadystatechange = function(){
                if (this.readyState === 4) {
                    console.log("start");
                }
                origOnreadystatechange();
                if (this.readyState === 4) {
                    console.log("end");
                }
            };
        }
        // execute the original "send" method
        origSend.call(this, data);
    };
})();

Usage (just like a usual XMLHttpRequest):

var xhr = new XMLHttpRequest();
xhr.open("POST", "test.html", true);
xhr.onreadystatechange = function(){
    if (xhr.readyState === 4) {
        console.log("will be printed between 'start' and 'end'");
    }
};
xhr.send();