Message passing from Background script to Content script behaving weidr

[Piyush Khemka]

I am trying to implement message passing from background script to content script

manifest.json

"background": {
        "scripts": ["background.js"],
        "persistent": false
    },


    "content_scripts" : [
    {
        "matches" : [ "<all_urls>" ],
        "js": ["hello.js"],
        "css": [ "hello.css" ]
    }

background.js

    function showpopup()
{


  chrome.tabs.query({active: true, currentWindow: true}, function(tabs) {
  chrome.tabs.sendMessage(tabs[0].id, {greeting: "hello"}, function(response) {
        console.log(response.farewell);
      });
    });
}


chrome.alarms.onAlarm.addListener(showpopup);

content script js aka hello.js

chrome.runtime.onMessage.addListener(
  function(request, sender, sendResponse) {
    console.log(sender.tab ?
                "from a content script:" + sender.tab.url :
                "from the extension");
    if (request.greeting == "hello") {

      alert("Hello World");
      sendResponse({farewell: "goodbye"});
}
  });

I am getting the error Error in event handler for (unknown): TypeError: Cannot read property 'farewell' of undefined

 I am setting an alarm in the background page using a chrome.alarms.create command. I noticed something interesting. Some pages are throwing a popup while others aren't. 

Seems like I am able to get a popup on the page if I refresh a particular tab.(I have around 10 open) On opening or refreshing a new tab, I am getting a popup whereas an old tab doesn't show a popup. Does anyone know how to correct this? 

Also is there a way to match chrome:// and newtab pages as well in the manifest? I tried including chrome:// but that didn't work as intended.

[PhistucK]

If you change -

 chrome.tabs.sendMessage(tabs[0].id, {greeting: "hello"}, function(response) {

        console.log(response.farewell);
To -

 chrome.tabs.sendMessage(tabs[0].id, {greeting: "hello"}, function(response) {

        console.log(arguments);

Does it print anything interesting?

☆PhistucK

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[Piyush Khemka]

Prints

[ ] for old tabs

[object] for newly opened tabs or refreshed tabs

☆PhistucK

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