How to overwrite success function through jQuery ajaxSend event

What you are trying to accomplish cannot be accomplished with ajaxSend. The problem is that it ajaxSendapparently works with a copy of the original objects xhrand options, therefore, the changes will have no effect. You can easily verify 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"); }
});

ajaxSend . "" jQuery AJAX:

// 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);
    };
})();

$.ajax, :

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

, jQuery AJAX (, $.get() .load()) $.ajax, AJAX, jQuery ( , ...).

---

- "" JavaScript, XMLHttpRequest.prototype. , IE : XMLHttpRequest ActiveXObject.

(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);
    };
})();

( 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();