javascript - Display alert only once

Question

I have this code that shows alert when it's 30 minutes before event in calendar but I want to show it only once when user comes to page (in that 30min). Now it shows on every page refresh and also on calendar refresh (in that 30min) because it is set to refresh events after period of time. How to display this alert just once?

var mili = event.start.getTime() - now.getTime();
if(mili < 1800000 && mili > 0){    
$('.alert').dialog({ 
        buttons: [{ 
        text: "OK", 
          click: function() { 
            $( this ).dialog( "close" ); 
          }
        }]
    });
}

Answer 1

You can use localStorage (not compatible with older browsers):

<script type="text/javascript">
    var alerted = localStorage.getItem('alerted') || '';
    if (alerted != 'yes') {
     alert("My alert.");
     localStorage.setItem('alerted','yes');
    }
</script>

Or you can use cookies, give a look to this answer for a full example code: https://stackoverflow.com/a/21567127/3625883