chrome extension – sendResponse not waiting for async function [duplicate]

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.

Solution

Use a standard function callback and a nested async 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
  }
});

Or 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';
}