Can I do synchronous cross-domain communicating with window.postMessage?

The postMessage() function is asynchronous, meaning it will return immediately. So you can not do synchronous communication with it.

In your example, the posted message will vanish in the void, because there is no listener for the message event at the time the postMessage() function is executed.

If you would load the iframe first and call postMessage() afterwards, then there could be an timing issue, maybe. (From my experience there is none, the parent code is always execute first, but I am not sure about this point.)

Below is my solution for the problem of not knowing exactly when the iframe will be ready.

In the parent window:

1. Load the iframe (this is asynchronous, too)
2. Set up message listener
3. Post message to the iframe (just trying here)
4. Wait for more messages to come

In the iframe:

1. Set up message listener
2. Post message to the parent window (just trying here)
3. Wait for more messages to come

Whoever receives the first message from the other side then starts the real communication.

In my experience, the message from the parent to the iframe always gets lost, so the communication starts when the parent receives the message from the iframe. But in this setup it is not important which one starts first.