Content script not receiving message from background script (Mv3)

Question

I made this simple messaging example chrome extension, where a message is supposed to be sent from the background script to the content script. Unfortunately, it seems the content script doesn't receive the message.

Background script:

// background.js

function sendMessage(tabId, hostname) {
    console.log("Sending message to tabId: ", tabId)
    chrome.tabs.sendMessage(tabId, {hostname: hostname}, (resp) => {console.log("response: ", resp)});
}

chrome.tabs.onUpdated.addListener((tabId, changeInfo, tab) => {
    if (changeInfo.url) {
        console.log(changeInfo.url)
        var hostname = new URL(changeInfo.url).hostname;
        sendMessage(tabId, hostname)

    }
});

Content script:

// content.js

console.log("injected");

function logMessage(message) {
    console.log("Message from background: ", message)
}

chrome.runtime.onMessage.addListener(
    (request, sender, sendResponse) => {
        logMessage(request.hostname)
    }
);

Manifest (v3):

// manifest.json

{
  "name": "Messaging test",
  "description": "",
  "version": "1.0",
  "manifest_version": 3,

  "background": {
    "service_worker": "background.js"
  },
  "permissions": ["tabs"],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>",
        "https://*/*",
        "http://*/*"
      ],
      "js": ["content.js"]
    }
  ]
}

I made sure to reload the extension and use a new tab for testing.

Here is the dev console output from the background script: dev console output of background script

And here is the dev console output from the content script (injected into google.com): dev console output of content script

So, the content script gets injected, but doesn't receive the message from the background script. I recall this working in manifest v2, so I'm not sure what's wrong. Any ideas?

Answer 1

Content scripts run after DOMContentLoaded by default, but onUpdated event is triggered when the tab starts loading a URL, so when sendMessage is called there's no content script yet in this tab.

Solution: specify "run_at": "document_start" so the content script already runs when onUpdated reports changeInfo.url:

  "content_scripts": [{
    "matches": ["<all_urls>"],
    "js": ["content.js"],
    "run_at": "document_start"
  }],

Another solution would be to reverse the direction of communication and let the content script call sendMessage while the background script would return the data in onMessage, see messaging.