Chrome / Firefox extension - Content script not listening for messages

Question

I am developing an extension for both Chrome and Firefox, and I have come across a problem.

Basically, I am trying to get a content script to listen to messages using chrome.runtime.onMessage.addListener(...), however it doesn't seem to work.

I tested it by sending a message from the content script. The background script (ml.js) had a listener that worked fine, but the lsitener in the content script just didn't get the message.

You can view the code in this Gist (or below).

manifest.json:

{
    "manifest_version": 2,
    "name": "Messaging Extension",
    "version": "1.0.0",
    "background": {
        "scripts": ["ml.js"]
    },
    "content_scripts": [
        {
            "matches": ["*://*.google.co.uk/*"],
            "js": ["cs.js"],
            "run_at": "document_end"
        }
    ]
}

ml.js:

// When receive message...
chrome.runtime.onMessage.addListener(function(message) {
    if (message.key) {
        console.log('ML: First message received')
        // Send another message
        chrome.runtime.sendMessage({
            'foo': 'bar'
        })
    }
})

cs.js:

// Send message to ml.js
chrome.runtime.sendMessage({
    'key': 'value'
})
chrome.runtime.onMessage.addListener(function(message) {
    console.log('CS: Second message received')
})

When tested in Firefox (by loading the add-on at about:debugging and then visiting Google), cs.js sent the message, and ml.js logged the message to the console, however cs.js didn't log the message.

I'll appreciate some help, thanks!

Unformed

Are you expecting the content script to receive the message it sent? This does not happen. A message sent by a script is not received by the same script. – Bartender

Answer 1

Using runtime.sendMessage() (messages to background scripts):

The runtime.sendMessage() (Chrome/Firefox) method is used to send messages to scripts that are running in the background context (background scripts, popup scripts, etc). Even when it is used to send messages from a script that is in the background context (e.g. to communicate between a popup script and a background script), it will be received by other currently listening background context scripts, but not the script that is sending it.

To quote the Google Chrome runtime.sendMessage() documentation (emphasis mine):

If sending to your extension, the runtime.onMessage event will be fired in every frame of your extension (except for the sender's frame)...

Sending a message to your content script (tabs.sendMessage())

If you are wanting to send a message from your background script to your content script you should be using tabs.sendMessage() (Chrome/Firefox). Alternately, you can use the connect() methods in runtime and tabs, which then provide you with a port (Chrome/Firefox).

References:

• Message Passing (Chrome)
• Communicating with background scripts (Firefox)

Bartender

The sending of the message from the content script was just for demonstration purposes, but I see now that it was unclear (also I didn't know that you cannot receive a message you send from the same script). I have updated the question (and Gist) with new code demonstrating the same error. Thanks for the help though! – Unformed

Basically, even if the message is sent from ml.js using runtime.sendMessage, the content script will not receive it using runtime.onMessage. – Unformed

@Druzion, That is the expected behavior. runtime.sendMessage() is for sending messages to scripts in the background script context (e.g. background scripts popup scripts, etc.), not content scripts. If you want to send a message to your content script, you should use tabs.sendMessage(). Please see the documentation linked in the the "References" above. – Bartender

@Druzion, No problem. I'm glad I was able to help. In re-reading the Firefox runtime.sendMessage() documentation, I see that it could be a bit more clear. I've added it to my list of pages which I should update. – Bartender

Yes, I was reading runtime.sendMessage as the method to send messages to content scripts, instead of from them. – Unformed