Message callback returns a value infrequently - Chrome Extension

Question

I'm building a chrome extension which communicates with a nodejs server through websockets. The point of it is to track browsing history with content. It all seems to work, but occasionally (30% of the time) the callback in a function passed to onMessage.addListener doesn't fire correctly. Let me show you the code:

background.js

var socket = io('http://localhost:3000/');

var tabLoad = function (tab) {
    socket.emit('page load', tab);
};

var tabUpdate = function (tabid, changeinfo, tab) {
    var url = tab.url;
    if (url !== undefined && changeinfo.status == "complete") {
        tab.user_agent = navigator.userAgent;
        tab.description = '';
        tab.content = '';

        socket.emit('insert', tab);
    }
};

socket.on('inserted', function(page){
    socket.emit('event', 'Requesting page content\n');
    //page = {tab: page, id: docs._id};
    chrome.tabs.sendMessage(page.tab_id, {requested: "content", page: page}, function(data) {
        socket.emit('content', data);
    });

});

try {
    chrome.tabs.onCreated.addListener(tabLoad);
    chrome.tabs.onUpdated.addListener(tabUpdate);
} catch(e) {
    alert('Error in background.js: ' + e.message);
}

content script - public.js

var messageHandler = function(request, sender, sendContent) {
    if (request.requested == "content") {
        var html = document.getElementsByTagName('html')[0].innerHTML;
        var data = {
            content: html,
            page: request.page
        };
        sendContent(data);
        return true;
    }
};

chrome.extension.onMessage.addListener(messageHandler);

The problem is that sometimes data in sendContent is undefined, while sometimes it is alright. Any ideas how to debug this or what i'm doing wrong?

I've tried replacing document.getElementsByTagName('html')[0].innerHTML with a hardcoded 'test' string, but that didn't help.

Pages like youtube/wikipedia seem to never work, while facebook/google works.

Edit: The sendContent callback does fire 100% of the time it's just that the data passed to it is undefined.

Edit: Here's the manifest file

{
    "manifest_version": 2,

    "name": "Socket test",
    "description": "sockets are cool",
    "version": "1.0",

    "permissions": [
        "http://st-api.localhost/",
        "http://localhost:3000/",
        "tabs",
        "background",
        "history",
        "idle",
        "notifications"
    ],
    "content_scripts": [{
        "matches": ["*://*/"],
        "js": ["public/public.js"]
        //"run_at": "document_start"
    }],
    //"browser_action": {
    //    "default_icon": "logo.png",
    //    "default_popup": "index.html"
    //},
    "background": {
        //"page" : "background.html",
        "scripts": ["socket-io.js", "background.js"],
        "persistent": true
    }
}

Answer 1

First off, your understanding that sendContent is executed 100% of the time is wrong.

As established in the comments, the sendMessage callback also gets executed when there was an error; and this error is, in your case, "Receiving end does not exist"

---

The error lies in your manifest declaration of the content script. A match pattern "*://*/" will only match top-level pages on http and https URIs. I.e. http://example.com/ will match, while http://example.com/test will not.

The easiest fix is "*://*/*", but I would recommend the universal match pattern "<all_urls>".

---

With that fixed, there are still a couple of improvements to your code.

• Replace chrome.extension.onMessage (which is deprecated) and use chrome.runtime.onMessage
• Modify the sendMessage part to be more resilient, by checking for chrome.runtime.lastError. Despite the wide permission, Chrome still won't inject any content scripts into some pages (e.g. chrome:// pages, Chrome Web Store)
• Make sure you use "run_at" : "document_start" in your content script, to make sure onUpdated with "complete" is not fired before your script is ready.