<iframe id="target-iframe" src="somepage.html"></iframe>
​
<script>
    var iframe = document.getElementById('target-iframe');
    if (iframe) {
​
        window.addEventListener('message', function (evt) {
            var value;
            if (evt.data.indexOf('variable:') === 0) {
                value = evt.data.replace('variable:', '');
                console.log(value); // do sth with the value
            }
        });
​
        iframe.contentWindow.postMessage('getVariable', 'http://example.com')
    }
</script>
​
<!-- na stronie która jest w iframe (w tym przypadku: somepage.html) -->
<script>
    window.addEventListener('message', function (evt) {
        if (evt.data === 'getVariable' && window.parent && window.parent.postMessage) {
            window.parent.postMessage('variable:' + yourVariable);
        }
    });
</script>