Capture XHR requests and alter the response body

Question

Capture XHR requests and alter the response body

I'm in the process of developing a script that serves as a proxy/wrapper for the conventional XMLHttpRequest object. This will allow me to intercept it, alter the responseText, and then return it back to the original onreadystatechange event.

The main objective is to stop the XMLHttpRequest if the data the application is attempting to fetch is already stored locally, and instead feed the cached data into the app's success/failure callback functions. It's important to note that I have no control over the current AJAX callback methods used by the app.

Initially, I attempted the following approach:

var send = XMLHttpRequest.prototype.send;
XMLHttpRequest.prototype.send = function(data){
   //Do some modifications here to change the responseText
   send.call(this, data);
};

Unfortunately, it turns out that the responseText property is read-only.

Afterward, I took a step back and tried constructing my own complete native proxy to XMLHttpRequest, essentially recreating my versions of the native functions. This concept was similar to what is discussed in this article...

However, things quickly became convoluted, and I continued to struggle with returning the modified data to the original onReadyStateChange method.

Any recommendations or insights on how to achieve this? Is this goal even feasible?

javascript ajax xmlhttprequest

Answer 1

Answer №1

//
// Improved functionality for XMLHttpRequest responseText property
//

// Code snippet for Firefox and IE8+
var accessor = Object.getOwnPropertyDescriptor(XMLHttpRequest.prototype, 'responseText');

Object.defineProperty(XMLHttpRequest.prototype, 'responseText', {
  get: function() {
    console.log('get responseText');
    return accessor.get.call(this);
  },
  set: function(str) {
    console.log('set responseText: %s', str);
    //return accessor.set.call(this, str);
  },
  configurable: true
});


// Code snippet for Chrome and Safari (where accessor is null)
var rawOpen = XMLHttpRequest.prototype.open;

XMLHttpRequest.prototype.open = function() {
  if (!this._hooked) {
    this._hooked = true;
    setupHook(this);
  }
  rawOpen.apply(this, arguments);
}

function setupHook(xhr) {
  function getter() {
    console.log('get responseText');

    delete xhr.responseText;
    var ret = xhr.responseText;
    setup();
    return ret;
  }

  function setter(str) {
    console.log('set responseText: %s', str);
  }

  function setup() {
    Object.defineProperty(xhr, 'responseText', {
      get: getter,
      set: setter,
      configurable: true
    });
  }
  setup();
}

Answer 2

//
// Improved functionality for XMLHttpRequest responseText property
//

// Code snippet for Firefox and IE8+
var accessor = Object.getOwnPropertyDescriptor(XMLHttpRequest.prototype, 'responseText');

Object.defineProperty(XMLHttpRequest.prototype, 'responseText', {
  get: function() {
    console.log('get responseText');
    return accessor.get.call(this);
  },
  set: function(str) {
    console.log('set responseText: %s', str);
    //return accessor.set.call(this, str);
  },
  configurable: true
});


// Code snippet for Chrome and Safari (where accessor is null)
var rawOpen = XMLHttpRequest.prototype.open;

XMLHttpRequest.prototype.open = function() {
  if (!this._hooked) {
    this._hooked = true;
    setupHook(this);
  }
  rawOpen.apply(this, arguments);
}

function setupHook(xhr) {
  function getter() {
    console.log('get responseText');

    delete xhr.responseText;
    var ret = xhr.responseText;
    setup();
    return ret;
  }

  function setter(str) {
    console.log('set responseText: %s', str);
  }

  function setup() {
    Object.defineProperty(xhr, 'responseText', {
      get: getter,
      set: setter,
      configurable: true
    });
  }
  setup();
}

Answer 3

Answer №2

This clever script efficiently intercepts data before it is sent via XMLHttpRequest.prototype.send

<script>
(function(customSend) { 

        XMLHttpRequest.prototype.send = function(data) { 

            this.addEventListener('readystatechange', function() { 

            }, false); 

            console.log(data); 
            alert(data); 

        }; 

})(XMLHttpRequest.prototype.send);
</script>

Answer 4

This clever script efficiently intercepts data before it is sent via XMLHttpRequest.prototype.send

<script>
(function(customSend) { 

        XMLHttpRequest.prototype.send = function(data) { 

            this.addEventListener('readystatechange', function() { 

            }, false); 

            console.log(data); 
            alert(data); 

        }; 

})(XMLHttpRequest.prototype.send);
</script>

Answer 5

Answer №3

In my view, a more contemporary approach to intercepting the response of XMLHttpRequest involves extending the original XMLHttpRequest object and overriding it within the window namespace:

const { interceptXhrResponse } = (function () {
  let interceptionRules = [];

  /**
   * Function for intercepting responses based on URL patterns
   * @param {RegExp} urlPattern - Regular expression to match the (canonicalized) URL
   * @param {Function} responseHandler - Handler function for intercepted response
   */
  function interceptXhrResponse(urlPattern, responseHandler) {
    interceptionRules.push({ urlPattern, responseHandler });
  }

  // Find specific handler for the URL and modify response accordingly
  function handleInterceptedResponse(response, url) {
    const interceptionRule = interceptionRules.find(({ urlPattern }) =>
      urlPattern.test(url)
    );

    if (interceptionRule) {
      const { responseHandler } = interceptionRule;
      return responseHandler(response, url);
    }

    return response;
  }

  const OriginalXMLHttpRequest = window.XMLHttpRequest;

  class XMLHttpRequest extends OriginalXMLHttpRequest {
    get responseText() {
      if (this.readyState !== 4) {
        return super.responseText;
      }

      return handleInterceptedResponse(super.responseText, this.responseURL);
    }

    get response() {
      if (this.readyState !== 4) {
        return super.response;
      }

      return handleInterceptedResponse(super.response, this.responseURL);
    }
  }

  window.XMLHttpRequest = XMLHttpRequest;

  return { interceptXhrResponse };
})();

The code above introduces the interceptXhrResponse function which allows users to define a URL pattern using a regular expression along with a response handler. The handler can be used to manipulate the response as needed.

For instance:

interceptXhrResponse(/.+/, (response, url) => {
  return `Response of ${url}: Intercepted. Original response length: ${String(response).length}`
})

Then we can create an XMLHttpRequest instance:

const xhr = new XMLHttpRequest()
xhr.open('GET', 'https://stackoverflow.com/404')
xhr.send()
xhr.onloadend = () => {
  console.log(xhr.responseText)
}

Output:

Response of https://stackoverflow.com/404: Intercepted. Original response length: 63486

Answer 6

In my view, a more contemporary approach to intercepting the response of XMLHttpRequest involves extending the original XMLHttpRequest object and overriding it within the window namespace:

const { interceptXhrResponse } = (function () {
  let interceptionRules = [];

  /**
   * Function for intercepting responses based on URL patterns
   * @param {RegExp} urlPattern - Regular expression to match the (canonicalized) URL
   * @param {Function} responseHandler - Handler function for intercepted response
   */
  function interceptXhrResponse(urlPattern, responseHandler) {
    interceptionRules.push({ urlPattern, responseHandler });
  }

  // Find specific handler for the URL and modify response accordingly
  function handleInterceptedResponse(response, url) {
    const interceptionRule = interceptionRules.find(({ urlPattern }) =>
      urlPattern.test(url)
    );

    if (interceptionRule) {
      const { responseHandler } = interceptionRule;
      return responseHandler(response, url);
    }

    return response;
  }

  const OriginalXMLHttpRequest = window.XMLHttpRequest;

  class XMLHttpRequest extends OriginalXMLHttpRequest {
    get responseText() {
      if (this.readyState !== 4) {
        return super.responseText;
      }

      return handleInterceptedResponse(super.responseText, this.responseURL);
    }

    get response() {
      if (this.readyState !== 4) {
        return super.response;
      }

      return handleInterceptedResponse(super.response, this.responseURL);
    }
  }

  window.XMLHttpRequest = XMLHttpRequest;

  return { interceptXhrResponse };
})();

The code above introduces the interceptXhrResponse function which allows users to define a URL pattern using a regular expression along with a response handler. The handler can be used to manipulate the response as needed.

For instance:

interceptXhrResponse(/.+/, (response, url) => {
  return `Response of ${url}: Intercepted. Original response length: ${String(response).length}`
})

Then we can create an XMLHttpRequest instance:

const xhr = new XMLHttpRequest()
xhr.open('GET', 'https://stackoverflow.com/404')
xhr.send()
xhr.onloadend = () => {
  console.log(xhr.responseText)
}

Output:

Response of https://stackoverflow.com/404: Intercepted. Original response length: 63486

Answer 7

Answer №4

Instead of going overboard with your step-back, you have the option to create your own getter for XMLHttpRequest. Learn more about properties here.

Object.defineProperty(XMLHttpRequest.prototype,"myResponse",{
  get: function() {
    return this.responseText+"custom update"; // customize as needed
  }
});

Here's how you can use it:

var xhr = new XMLHttpRequest();
...
console.log(xhr.myResponse); // xhr.responseText+"custom update"

Keep in mind that on modern browsers, you can utilize xhr.onload. Find out more at XMLHttpRequest2 tips.

Answer 8

Instead of going overboard with your step-back, you have the option to create your own getter for XMLHttpRequest. Learn more about properties here.

Object.defineProperty(XMLHttpRequest.prototype,"myResponse",{
  get: function() {
    return this.responseText+"custom update"; // customize as needed
  }
});

Here's how you can use it:

var xhr = new XMLHttpRequest();
...
console.log(xhr.myResponse); // xhr.responseText+"custom update"

Keep in mind that on modern browsers, you can utilize xhr.onload. Find out more at XMLHttpRequest2 tips.

Answer 9

Answer №5

Previously, the solution that was working for me with Twitter suddenly stopped working for some unknown reason. Here is an alternative solution that I have found to be effective:

        var open_prototype = XMLHttpRequest.prototype.open
        unsafeWindow.XMLHttpRequest.prototype.open = function() {
            this.addEventListener('readystatechange', function(event) {
                if ( this.readyState === 4 ) {
                    var response = event.target.responseText.replaceAll("a", "b");
                    Object.defineProperty(this, 'response', {writable: true});
                    Object.defineProperty(this, 'responseText', {writable: true});
                    this.response = this.responseText = response;
                }
            });
            return open_prototype.apply(this, arguments);
        };

To filter based on URL, you can use event.target.responseURL

Answer 10

Previously, the solution that was working for me with Twitter suddenly stopped working for some unknown reason. Here is an alternative solution that I have found to be effective:

        var open_prototype = XMLHttpRequest.prototype.open
        unsafeWindow.XMLHttpRequest.prototype.open = function() {
            this.addEventListener('readystatechange', function(event) {
                if ( this.readyState === 4 ) {
                    var response = event.target.responseText.replaceAll("a", "b");
                    Object.defineProperty(this, 'response', {writable: true});
                    Object.defineProperty(this, 'responseText', {writable: true});
                    this.response = this.responseText = response;
                }
            });
            return open_prototype.apply(this, arguments);
        };

To filter based on URL, you can use event.target.responseURL

Capture XHR requests and alter the response body

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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