// this script exists on the *host site*
const iframe = document.createElement("iframe");
iframe.src = "https://us.com/iframe";
parent.appendChild(iframe);

// This script exists on the *host site*
function getSomeProperty()
{
    return new Promise(function (resolve, reject)
    {
        theIFrame.postMessage("getSomeProperty", "us.com")
        window.addEventListener("message", function (event)
        {
            // ... get the response, making sure its from us.com, etc.
            // This is just pseudo code, we don't create a new listener
            // on every call in our real code.
            resolve(answer);
        }
    });
}

const parent = window.parent;

// This event handler is in the *embedded iframe*
window.addEventListener("message", function (event)
{
    // This is being sent from a window other than
    // the one that created us, bail!
    if (event.window !== parent)
         return;

    // it is safe to respond...
}

const iframe = document.createElement("iframe");
iframe.src = `https://us.com/iframe?location=${window.location.href}`;