Message Passing - Confused by examples

Question

I've read the documentation over and over but I find it confusing.

What I'm trying to do is send a list of links to my background script to do some fetch requests, and then pass the response codes for those links to either popup.js or content-script.js so that I can highlight each link depending on its status

// popup.js
chrome.runtime.sendMessage({ message: filteredLinks }, (response) => {
    console.log("HTTP CODES -->", response.message);
    })

---

// background.js //
chrome.runtime.onMessage.addListener(
  (request, sender, sendResponse) => {
    let messReps = [];
    let rep = request.message;

    for (i = 0; i < rep.length; i++) {
      fetch(rep[i], {
        method: 'GET',
        mode: 'cors'
      })
        .then(response => messReps.push(response.status))
    }
    console.log("MESS REPS", messReps);
    sendResponse({ message: messReps })
    return true;
  });

When I log 'messReps' it shows all of the messages' response codes, but when I try to send them back via sendResponse() I end up with an empty array.

I thought that the 'return true;' statement was supposed to hold off on sending until the function was complete, so I'm not sure what I'm doing wrong.