Clash Royale CLAN TAG

How to detect in post.message null var?

Hello I need detect if child.php not send message to parent.php, I use the post.message .
I try with:

if (message == null), == false, == "", but not work ..


My code:

parent.php (get the message "work" from child.php if its open)

<button id="open-child-window-2">Open Child Window</button>

<script>

var __CHILD_WINDOW_HANDLE_2 = null;

$("#open-child-window-2").on('click', function() {
__CHILD_WINDOW_HANDLE_2 = window.open('child-2.php', '_blank');
});


window.addEventListener('message', function(e) {
ProcessChildMessage_2(e.data);
} , false);


function ProcessChildMessage_2(message) {
alert(message);
}

</script>


child.php (send to parent.php the message "work" every 0.5 seconds)

<script>

setInterval(function(){

window.opener.postMessage("work", "*");

},500);

</script>


I need detect if chiel.php not send "work" to parent.php, for example:

Thanks you so much

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