Vega Vega - 1 year ago 204
Javascript Question

chrome.notification.create with chrome.notification.onClicked in a Firefox WebExtension add-on

I have a problem understanding the documentation for the WebExtensions


Ultimately, I'm trying to get the text of the notification copied to the clipboard when you click on it. However, right now I am having a problem understanding the callback thing, or where I have to insert the

At the moment, I don't know why the
listener does nothing.

My code (all the code needed to demonstrate the problem as a WebExtension Firefox add-on):


"description": "Test Webextension",
"manifest_version": 2,
"name": "Σ",
"version": "1.0",

"permissions": [

"background": {
"scripts": ["background.js"]


'use strict';

function logURL(requestDetails) {
chrome.notifications.onClicked.addListener(function() {
console.log("TEST TEST");

function notify(notifyMessage) {
var options = {
type: "basic",
iconUrl: chrome.extension.getURL("icons/photo.png"),
title: "",
message: notifyMessage
chrome.notifications.create("ID123", options);

logURL, {
urls: ["<all_urls>"]

Answer Source

First, you need to be testing this in Firefox 47.0+, as support for chrome.notifications.onClicked() was added in version 47.0. While this is probably not your problem, it is one contributing possibility.

There are multiple issues with your code. Some are in your code, but primarily you are running into a Firefox bug.

Firefox Bug:
Your primary issue is that you are running into a Firefox bug where Firefox gets confused if you try to create notifications too rapidly. Thus, I have implemented a notification queue and rate limited the creation of notifications. What is "too rapidly" is probably both OS and CPU dependent, so you are best off erroring on the side of caution and set the delay between calls to chrome.notifications.create() to a higher value. in the code below, the delay is 500ms. I have added a note regarding this issue in the chrome.notifications.create() page on MDN and on the Chrome incompatibilities page.

Adding multiple copies of the same listener:
The main thing that you are doing wrong in your code is that you are adding an anonymous function as a listener, using chrome.notifications.onClicked.addListener(), multiple times to the same event. This is a generic issue with event handlers. When you use an anonymous function it is a different actual function each time you are trying to add it, so the same functionality (in multiple identical functions) gets added multiple times. You should not be adding functions, which do the exact same thing, multiple times to the same event. Doing so is almost always an error in your program and results in unexpected operation.

In this case, the multiple functions would have ended up outputing multiple lines of TEST TEST to the console each time the user clicked on a notification. The number of lines output per click would increase by one for each web request which resulted in a call to logURL.

The way to prevent doing this is to be sure to add the listener only once. If you are using an anonymous function, you can only do this by being sure you only execute the addListener (or addEventlistener) once (usually by only adding the listener from your main code (not from within a function), or from a function that is only called once. Alternately, you can name/define your listener function directly within the global scope (or other scope accessible to all places where you try to add the listener) (e.g. function myListener(){...}). Then, when you are adding myListener you are always referring to the same exact function which JavaScript automatically prevents you from adding in the same way to the same event more than once.

It should be noted that if you are trying to add a anonymous function as a listener from another listener, you are almost always doing something wrong. Adding copies of identical anonymous listeners multiple times to the same event is a common error.

Access to the notification text:
While you do not implement anything regarding using the text of the notification, you state that you want to add the text of the notification to the clipboard when the user clicks on the notification. You can not obtain the notification text from any portion of the chrome.notifications API. Thus, you have to store that information yourself. The code below implements an Object to do that so the text can be accessed in the chrome.notifications.onClicked() handler.

Example code:
The code below implements what I believe you desire. It is just creating and clicking the notification while having access to the notification text in the chrome.notifications.onClicked() listener. It does not implement the part about putting the text into the clipboard, as that was not actually implemented in the code in your Question. I have added liberal comments to the code to explain what is happening and provided quite a bit of console.log() output to help show what is going on. I have tested it in both Firefox Developer Edition (currently v51.0a2) and Google Chrome.

background.js (no changes to your manifest.json):

'use strict';
//* For testing, open the Browser Console
var isFirefox = window.InstallTrigger?true:false;
    if(isFirefox){  //Only do this in Firefox
        //Alert is not supported in Firefox. This forces the Browser Console open.
        //This abuse of a misfeature works in FF49.0b+, not in FF48
        alert('Open the Browser Console.');
    //alert throws an error in Firefox versions below 49
    console.log('Alert threw an error. Probably Firefox version below 49.');

//Firefox gets confused if we try to create notifications too fast (this is a bug in
//  Firefox).  So, for Firefox, we rate limit showing the notifications.
//  The maximum rate possible (minimum delay) is probably OS and CPU speed dependent.
//  Thus, you should error  on the side of caution and make the delay longer.
//  No delay is needed in Chrome.
var notificationRateLimit = isFirefox ? 500:0;//Firefox:Only one notification every 500m
var notificationRateLimitTimeout=-1; //Timeout for notification rate limit
var sentNotifications={};
var notificationsQueue=[];
var notificationIconUrl = chrome.extension.getURL("icons/photo.png");
function logURL(requestDetails) {
    //console.log('webRequest.onBeforeRequest URL:' + requestDetails.url);
    //NOTE: In Chrome, a webRequest is issued to obtain the icon for the notification. 
    //  If Chrome finds the icon, that webRequest for the icon is only issued twice.
    //  However, if the icon does not exist, then this sets up an infinite loop which
    //  will peg one CPU at maximum utilization.
    //  Thus, you should not notify for the icon URL.
    //  You should consider excluding from notification all URLs from within your
    //  own extension.
    if(requestDetails.url !== notificationIconUrl ){
        notify('webRequest URL: ' + requestDetails.url);
    //Your Original code in the Question:
    //Unconditionally adding an anonymous notifications.onClicked listener
    //  here would result in multiple lines of 'TEST TEST' ouput for each click
    //  on a notification. You should add the listener only once.

function notify(notifyMessage) {
    //Add the message to the notifications queue.
    console.log('Notification added to queue. message:' + notifyMessage);
    if(notificationsQueue.length == 1){
        //If this is the only notification in the queue, send it.
    //If the notificationsQueue has additional entries, they will get
    //  shown when the current notification has completed being shown.

function showNotificationQueueWithRateLimit(){
        //There is no current delay active, so immediately send the notification.
    //If there is a delay active, we don't need to do anything as the notification
    //  will be sent when it gets processed out of the queue.

function showNextNotification() {
    notificationRateLimitTimeout=-1; //Indicate that there is no current timeout running.
    if(notificationsQueue.length === 0){
        return;  //Nothing in queue
    //Indicate that there will be a timeout running.
    //  Neeed because we set the timeout in the notifications.create callback function.
    //Get the next notification from the queue
    let notifyMessage = notificationsQueue.shift();
    console.log('Showing notification message:' + notifyMessage);
    //Set our standard options
    let options = {
        type: "basic",
        //If the icon does not exist an error is generated in Chrome, but not Firefox.
        //  In Chrome a webRequest is generated to fetch the icon. Thus, we need to know
        //  the iconUrl in the webRequest handler, and not notify for that URL.
        iconUrl: notificationIconUrl,
        title: "",
        message: notifyMessage
    //If you want multiple notifications shown at the same time, your message ID must be
    //  unique (at least within your extension).
    //Creating a notification with the same ID causes the prior notification to be
    //  destroyed and the new one created in its place (not just the text being replaced).
    //Use the following two lines if you want only one notification at a time.  If you are
    //  actually going to notify on each webRequest (rather than doing so just being a way
    //  to test), you should probably only have one notification as they will rapedly be
    //  off the screen for many pages.
    //let myId = 'ID123';
    //If you want multiple notifications without having to create a unique ID for each one,
    //  then let the ID be created for you by using the following line:
        //In this callback the notification has not necessarily actually been shown yet,
        //  just that the notification ID has been created and the notification is in the
        //  process of being shown.
        console.log('Notification created, id=' + id + ':: message:' + notifyMessage);
        //Remember the text so we can get it later
        sentNotifications[id] = {
            message: notifyMessage
        //Show the next notification in the FIFO queue after a rate limiting delay
        //  This is called unconditionally in order to start the delay should another
        //  notification be queued, even if one is not in the queue now.
        notificationRateLimitTimeout = setTimeout(showNextNotification

function logIfError(){
        let message =chrome.runtime.lastError.message;
        console.log('Error: ' + message);

    logURL, {
        urls: ["<all_urls>"]

//Add the notifications.onClicked anonymous listener only once:
//  Personally, I consider it better practice to use a named function that
//  is defined in the global scope. Doing so prevents inadvertantly adding
//  it multiple times. Although, your code should be written such that you 
//  don't do that anyway.
chrome.notifications.onClicked.addListener(function(id) {
    //We can not get the notification text from here, just the ID.  Thus, we
    //  have to use the text which was remembered.
    console.log('Clicked notification message text: ', sentNotifications[id].message);
    //In Firefox the notification is automatically cleared when it is clicked.
    //  If you want the same functionality in Chrome, you will need to clear() it
    //  yourself: 
    //Always do this instead of only when not in Firefox so that it remains consistent
    //  Even if Firefox changes to match Chrome.
    //This is the last place we use the text of the notification, so we delete it
    //  from sentNotifications so we don't have a memory leak.
    delete sentNotifications[id];

//Test the notifications directly without the need to have webRequests:
notify('Background.js loaded');
notify('Second notification');

In the process of working on this, I found multiple incompatibilities between Chrome and Firefox. I am in the process of updating MDN to mention the incompatibilities in the documentation on MDN.

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