Getting HTTP Response Header in Firefox Addon

Question

I am trying to get the HTTP response by reading the response header. But I am not able to retrieve anything. What I am missing exactly? Here is my code in index.js

var {Cc, Ci} = require("chrome");
var httpRequestObserver =
{
   init: function() {
      var observerService = Cc["@mozilla.org/observer-service;1"].getService(Ci.nsIObserverService);
      observerService.addObserver(this, "http-on-examine-response", false);
   },

  observe: function(subject, topic, data) 
  {
    if (topic == "http-on-examine-response") {
         subject.QueryInterface(Ci.nsIHttpChannel);
            this.onExamineResponse(subject);
    }
  },
onExamineResponse: function (oHttp)
  {  
        console.log("Header :"+JSON.stringify(oHttp));
        try
       {
         var header_value = oHttp.getResponseHeader("<the_header_that_i_need>"); 
         // URI is the nsIURI of the response you're looking at 
         // and spec gives you the full URL string
         var url = oHttp.URI.spec;
       }
       catch(err)
       {
         console.log(err);
       }
   }
};

httpRequestObserver.init();

I get reference from this stackoverflow and several online blogs.:- Firefox add-on SDK: Get http response headers

Edited

I Checked for the values in console.

• subject is coming as ()
• data is coming as null

Answer 1

Do:

 oHttp = oHttp.QueryInterface(Ci.nsIHttpChannel);

in onExaminResponse then after this getRepsonseHeader should work

edit:

this works for me:

Services.obs.removeObserver(httpRequestObserver, "http-on-examine-response", false);
var httpRequestObserver =
{
   init: function() {
      Services.obs.addObserver(this, "http-on-examine-response", false);
   },

  observe: function(subject, topic, data) 
  {
         subject = subject.QueryInterface(Ci.nsIHttpChannel);
            this.onExamineResponse(subject);
  },
onExamineResponse: function (oHttp)
  {  
        //console.log("Header :"+JSON.stringify(oHttp));
         var url = oHttp.URI.spec;
         console.log('url:', url);

        try
       {
         // URI is the nsIURI of the response you're looking at 
         // and spec gives you the full URL string
         var header_value = oHttp.getResponseHeader("X-Firefox-Spdy"); 
         console.log('header_value:', header_value);
       }
       catch(err)
       {
         if (err.name == 'NS_ERROR_NOT_AVAILABLE') {
           console.log('this http request does not have header X-Firefox-Spdy')
         } else {
          console.error(err);
         }
       }
   }
};

httpRequestObserver.init();