How can I modify the XMLHttpRequest responsetext received by another function?

Question

I am trying to modify the responseText received by a function that I cannot modify. This function creates a XMLHttpRequest that I can attach to, but I have been unable to \"

Answer 1

I ran into the same problem when I was making a Chrome extension to allow cross origin API calls. This worked in Chrome. (Update: It doesn't work in the newest Chrome version).

delete _this.responseText;
_this.responseText = "Anything you want";

The snippet runs inside a monkeypatched XMLHttpRequest.prototype.send who is redirecting the requests to the extensions background script and replace all the properties on response. Like this:

// Delete removes the read only restriction
delete _this.response;
_this.response = event.data.response.xhr.response;
delete _this.responseText;
_this.responseText = event.data.response.xhr.responseText;
delete _this.status;
_this.status = event.data.response.xhr.status;
delete _this.statusText;
_this.statusText = event.data.response.xhr.statusText;
delete _this.readyState;
_this.readyState = event.data.response.xhr.readyState;

That didn't work in Firefox, but I found a solution that worked:

var test = new XMLHttpRequest();
Object.defineProperty(test, 'responseText', {
  configurable: true,
  writable: true,
});

test.responseText = "Hey";

That doesn't work in Chrome, but this work in both Chrome and Firefox:

var test = new XMLHttpRequest();
var aValue;
Object.defineProperty(test, 'responseText', {
  get: function() { return aValue; },
  set: function(newValue) { aValue = newValue; },
  enumerable: true,
  configurable: true
});

test.responseText = "Hey";

The last was copy past from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty

None of the solutions works in Safari. I tried to make a new XMLHttpRequest with writable properties, but it was not allowed to call open or send from it. I also tried this solution: https://stackoverflow.com/a/28513219/3717718. Unfortunately it produced the same error in Safari:

TypeError: Attempting to configurable attribute of unconfigurable property.