index.js

    webExtension.startup().then(api => {
      api.browser.runtime.onMessage.addListener((message, sender, sendReply) => {
        if ("method" in message && methods.indexOf(message.method) !== -1) {
          sendReply(this[message.method](message));
        }
      });

      console.log("webExtension.startup.then(), reason: ", reason);
      // Send "upgrade" message to WebExtension runtime
      if (reason === "upgrade") {
        console.log("sending upgrade message");
        api.browser.runtime.sendMessage("upgrade").then(reply => {
          console.log(reply);
        });
      }