An attempt to make postMessage async compatible, completely untested

postMessageAsyncAdapter.js

const registeredCallbacks = {};
const ORIGIN = location.origin;

function listener(event) {
    if (event.origin !== ORIGIN) {
        return;
    }
    let data = JSON.parse(event.data);
    let ticket = data.ticket;
    let payload = data.payload;
    if (data.error) {
        registeredCallbacks[ticket].reject(payload);
    } else {
        registeredCallbacks[ticket].resolve(payload);
    }
    delete registeredCallbacks[ticket];
}

export function registerListener() {
    window.addEventListener('message', listener);
}

export function unregisterListener() {
    window.removeEventListener('message', listener);
}

export function postMessageAsync(messageName, data) {
    return new Promise((resolve, reject) => {
        let ticket = new Date().getTime();
        let message = {
            messageName,
            data,
            ticket,
        };
        registeredCallbacks[ticket] = { resolve, reject };
        window.postMessage(message, ORIGIN);
    });
}