How can I intercept XMLHttpRequests from a Greasemonkey script?

The accepted answer is almost correct, but it could use a slight improvement:

(function(open) {    XMLHttpRequest.prototype.open = function() {        this.addEventListener("readystatechange", function() {            console.log(this.readyState);        }, false);        open.apply(this, arguments);    };})(XMLHttpRequest.prototype.open);

Prefer using apply + arguments over call because then you don't have to explicitly know all the arguments being given to open which could change!

How about modifying the XMLHttpRequest.prototype.open or send methods with replacements which set up their own callbacks and call the original methods? The callback can do its thing and then call the callback the original code specified.

In other words:

XMLHttpRequest.prototype.realOpen = XMLHttpRequest.prototype.open;var myOpen = function(method, url, async, user, password) {    //do whatever mucking around you want here, e.g.    //changing the onload callback to your own version    //call original    this.realOpen (method, url, async, user, password);}  //ensure all XMLHttpRequests use our custom open methodXMLHttpRequest.prototype.open = myOpen ;

Tested in Chrome 55 and Firefox 50.1.0

In my case I wanted to modify the responseText, which in Firefox was a read-only property, so I had to wrap the whole XMLHttpRequest object. I haven't implemented the whole API (particular the responseType), but it was good enough to use for all of the libraries I have.

Usage:

    XHRProxy.addInterceptor(function(method, url, responseText, status) {        if (url.endsWith('.html') || url.endsWith('.htm')) {            return "<!-- HTML! -->" + responseText;        }    });

Code:

(function(window) {    var OriginalXHR = XMLHttpRequest;    var XHRProxy = function() {        this.xhr = new OriginalXHR();        function delegate(prop) {            Object.defineProperty(this, prop, {                get: function() {                    return this.xhr[prop];                },                set: function(value) {                    this.xhr.timeout = value;                }            });        }        delegate.call(this, 'timeout');        delegate.call(this, 'responseType');        delegate.call(this, 'withCredentials');        delegate.call(this, 'onerror');        delegate.call(this, 'onabort');        delegate.call(this, 'onloadstart');        delegate.call(this, 'onloadend');        delegate.call(this, 'onprogress');    };    XHRProxy.prototype.open = function(method, url, async, username, password) {        var ctx = this;        function applyInterceptors(src) {            ctx.responseText = ctx.xhr.responseText;            for (var i=0; i < XHRProxy.interceptors.length; i++) {                var applied = XHRProxy.interceptors[i](method, url, ctx.responseText, ctx.xhr.status);                if (applied !== undefined) {                    ctx.responseText = applied;                }            }        }        function setProps() {            ctx.readyState = ctx.xhr.readyState;            ctx.responseText = ctx.xhr.responseText;            ctx.responseURL = ctx.xhr.responseURL;            ctx.responseXML = ctx.xhr.responseXML;            ctx.status = ctx.xhr.status;            ctx.statusText = ctx.xhr.statusText;        }        this.xhr.open(method, url, async, username, password);        this.xhr.onload = function(evt) {            if (ctx.onload) {                setProps();                if (ctx.xhr.readyState === 4) {                     applyInterceptors();                }                return ctx.onload(evt);            }        };        this.xhr.onreadystatechange = function (evt) {            if (ctx.onreadystatechange) {                setProps();                if (ctx.xhr.readyState === 4) {                     applyInterceptors();                }                return ctx.onreadystatechange(evt);            }        };    };    XHRProxy.prototype.addEventListener = function(event, fn) {        return this.xhr.addEventListener(event, fn);    };    XHRProxy.prototype.send = function(data) {        return this.xhr.send(data);    };    XHRProxy.prototype.abort = function() {        return this.xhr.abort();    };    XHRProxy.prototype.getAllResponseHeaders = function() {        return this.xhr.getAllResponseHeaders();    };    XHRProxy.prototype.getResponseHeader = function(header) {        return this.xhr.getResponseHeader(header);    };    XHRProxy.prototype.setRequestHeader = function(header, value) {        return this.xhr.setRequestHeader(header, value);    };    XHRProxy.prototype.overrideMimeType = function(mimetype) {        return this.xhr.overrideMimeType(mimetype);    };    XHRProxy.interceptors = [];    XHRProxy.addInterceptor = function(fn) {        this.interceptors.push(fn);    };    window.XMLHttpRequest = XHRProxy;})(window);