javascript: listen for postMessage events from specific iframe

You must listen on the global message event of the window object, but you can filter the source iframe using the source property of MessageEvent.

Example:

const childWindow = document.getElementById('test-frame').contentWindow;
window.addEventListener('message', message => {
    if (message.source !== childWindow) {
        return; // Skip message in this event listener
    }

    // ...
});