Chrome mv3 await sendMessage to background service worker [duplicate] - chrome-extension-manifest-v3

Question

This question already has answers here:
chrome.runtime.onMessage response with async await
(7 answers)
Closed 1 year ago.
I am having an issue of asynchronicity (I believe). sendResponse() in contentscript.js does not wait for getThumbnails() to return.
I am sending a message in popup.js:
chrome.tabs.sendMessage(tabs[0].id, {message: "get_thumbnails", tabUrl: tabs[0].url},
function (respThumbnails) {
const thumbUrl = respThumbnails.payload;
console.log("payload", thumbUrl)
}
);
Then, in contentscript.js I listen for this message:
chrome.runtime.onMessage.addListener(async function(request,sender,sendResponse) {
if(request.message === "get_thumbnails") {
const payload = await getThumbnails();
console.log("thumbPayload after function:", payload)
sendResponse({payload:payload});
}
});
async function getThumbnails() {
let tUrl = null;
var potentialLocations = [
{sel: "meta[property='og:image:secure_url']", attr: "content" },
{sel: "meta[property='og:image']", attr: "content" },
];
for(s of potentialLocations) {
if(tUrl) return
const el = document.querySelector(s.sel);
if(el) {
tUrl = el.getAttribute(s.attr) || null;
}
}
return tUrl;
};
But it is also possible that the problem is coming from my getThumnails() function, because most of the times, payload is null and not undefined. So getThumbnails() might return before it is completely executed.
If this is the case, I have no idea why...
I also tried this code for getThubnails():
async function getThumbnails() {
let x = await function() {
let tUrl = null;
var potentialLocations = [
{sel: "meta[property='og:image:secure_url']", attr: "content" },
{sel: "meta[property='og:image']", attr: "content" },
];
for(s of potentialLocations) {
if(tUrl) return
const el = document.querySelector(s.sel);
if(el) {
tUrl = el.getAttribute(s.attr) || null;
}
}
return tUrl;
}
return x;
};
But this does not work, it seems to break my code...

Answer 1

The callback of onMessage should return a literal true value (documentation) in order to keep the internal messaging channel open so that sendResponse can work asynchronously.
Problem
Your callback is declared with async keyword, so it returns a Promise, not a literal true value. Chrome extensions API doesn't support Promise in the returned value of onMessage callback until https://crbug.com/1185241 is fixed so it's just ignored, the port is immediately closed, and the caller receives undefined in response.
Solutions
Remove the async keyword from before (request, sender, sendResponse), then...
Solution 1
Call an async function that can be embedded as an IIFE:
chrome.runtime.onMessage.addListener((request, sender, sendResponse) => {
if (request.message === "get_thumbnails") {
(async () => {
const payload = await getThumbnails();
console.log("thumbPayload after function:", payload)
sendResponse({payload});
})();
return true; // keep the messaging channel open for sendResponse
}
});
Solution 2
Declare a separate async function and call it from the onMessage listener:
chrome.runtime.onMessage.addListener((msg, sender, sendResponse) => {
if (msg.message === "get_thumbnails") {
processMessage(msg).then(sendResponse);
return true; // keep the messaging channel open for sendResponse
}
});
async function processMessage(msg) {
console.log('Processing message', msg);
// .................
return 'foo';
}