Chrome extension to read HTTP response

I achieved capturing all HTTP requests and responses made by a website, by injecting a script to DOM. I injected injected.js to DOM using following script:

/**
 * code in inject.js
 * added "web_accessible_resources": ["injected.js"] to manifest.json
 */
var s = document.createElement('script');
s.src = chrome.extension.getURL('injected.js');
s.onload = function() {
    this.remove();
};
(document.head || document.documentElement).appendChild(s);

This would inject injected.js in website(s) that match “content_scripts” “matches” in manifest.json. Mention contentscript.js and inject.js in “js”.
Also, make sure you have mentioned the website in the “permissions” in manifest.json. See manifest.json at the end of answer.

Now, the code in injected.js which does the actual capturing of requests and responses is inspired from How we captured AJAX requests from a website tab with a Chrome Extension. Also see the comment section in that article.

The injected.js is as follows:

(function(xhr) {

    var XHR = XMLHttpRequest.prototype;

    var open = XHR.open;
    var send = XHR.send;
    var setRequestHeader = XHR.setRequestHeader;

    XHR.open = function(method, url) {
        this._method = method;
        this._url = url;
        this._requestHeaders = {};
        this._startTime = (new Date()).toISOString();

        return open.apply(this, arguments);
    };

    XHR.setRequestHeader = function(header, value) {
        this._requestHeaders[header] = value;
        return setRequestHeader.apply(this, arguments);
    };

    XHR.send = function(postData) {

        this.addEventListener('load', function() {
            var endTime = (new Date()).toISOString();

            var myUrl = this._url ? this._url.toLowerCase() : this._url;
            if(myUrl) {

                if (postData) {
                    if (typeof postData === 'string') {
                        try {
                            // here you get the REQUEST HEADERS, in JSON format, so you can also use JSON.parse
                            this._requestHeaders = postData;    
                        } catch(err) {
                            console.log('Request Header JSON decode failed, transfer_encoding field could be base64');
                            console.log(err);
                        }
                    } else if (typeof postData === 'object' || typeof postData === 'array' || typeof postData === 'number' || typeof postData === 'boolean') {
                            // do something if you need
                    }
                }

                // here you get the RESPONSE HEADERS
                var responseHeaders = this.getAllResponseHeaders();

                if ( this.responseType != 'blob' && this.responseText) {
                    // responseText is string or null
                    try {

                        // here you get RESPONSE TEXT (BODY), in JSON format, so you can use JSON.parse
                        var arr = this.responseText;

                        // printing url, request headers, response headers, response body, to console

                        console.log(this._url);
                        console.log(JSON.parse(this._requestHeaders));
                        console.log(responseHeaders);
                        console.log(JSON.parse(arr));                        

                    } catch(err) {
                        console.log("Error in responseType try catch");
                        console.log(err);
                    }
                }

            }
        });

        return send.apply(this, arguments);
    };

})(XMLHttpRequest);

For reference, my manifest.json is:

{
  "manifest_version": 2,

  "name": "Extension Name",
  "description": "Some Desc.",
  "version": "1.1",

  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  },
  "permissions": [
    "activeTab",
    "storage",
    "tabs",
    "*://website.com/*"
  ],
  "content_scripts": [
    {
      "matches": ["*://website.com/*"],
      "run_at": "document_start",
      "js": ["contentscript.js", "inject.js"]
    }
  ],
  "web_accessible_resources": ["injected.js"]
}

Hope this helps.