覆盖XMLHttpRequest.responseText

Question

I'll do my best to explain my problem but if I'm honest, I'm a little confused myself so I can't imagine it'll be much easier for you guys.

Right, I'm creating a script for a userscript for a website I frequent. What I'm trying to do is hijack any ajax requests, which I'm doing fine and then modify the responseText.

I can't seem to write to responseText, I can read it fine and it shows the response fine but I can't change it's value no matter what I try.

I get no errors in the console, I've left comments in my code to show what does log.

I was just going to scrap it but knowing me, I've missed something stupidly obvious and just can't see it.

Thanks in advance.

(function(send) { 
    XMLHttpRequest.prototype.send = function(data) { 
        this.addEventListener('readystatechange', function() { 
            if(typeof data == 'string'){
                if(data.indexOf('room.details_1') > -1){
                    if(this.readyState == 4 && this.status == 200){
                        console.log('Before: ' + JSON.parse(this.responseText).body.user.profile.username); // Shows NameNumber1
                        var temp = JSON.parse(this.responseText);
                        temp.body.user.profile.username = 'NameNumber2';
                        this.responseText = JSON.stringify(temp);
                        console.log('Temp: ' + temp.body.user.profile.username); // Shows NameNumber2
                        console.log('After: ' + JSON.parse(this.responseText).body.user.profile.username); // Shows NameNumber1 <-- This is the problem.
                        console.log(this); // Shows the XMLHttpRequest object, with the original responseText rather than the modified one.
                    }
                }
            }
        }, false);
        send.call(this, data);
    }; 
})(XMLHttpRequest.prototype.send);

Answer 1

I know this is 3 years too late, but I Have included it here for anyone else that comes across this thread. I've done this before... Here is a copy and paste directly from my script. You should be able to just change the responseText to be writable.

Object.defineProperty(this, "responseText", {writable: true});
this.responseText = '{"success":true}';

Answer 2

XMLHttpRequest.responseText is ReadOnly. This means there is no setter and therefore you can not modify its value. There is no workaround except you override XMLHttpRequest itself.

Specification

Edit Test for the suggestion to use Object.defineProperty to override responseText :

var xhr = new XMLHttpRequest();
Object.defineProperty( xhr, "responseText", { value: "test" });
xhr.responseText // returns ""

So this won't work either

Answer 3

this worked for me in chrome:

var request = new XMLHttpRequest();
delete request.responseText;
request.responseText = 'test';
request.responseText; // returns 'test'