How to make JS page redirect trigger before page load

I am using the following script to redirect visitors of a page to another page on first visit, however it loads the index.html page first, and then triggers the redirect. Can anyone point me in the direction of how I might trigger this script before the page loads?

<script type="text/javascript">
    function redirect(){
    var thecookie = readCookie('doRedirect');
    if(!thecookie){window.location = '/coming-soon.html';
    }}
    function createCookie(name,value,days){if (days){
    var date = new Date();date.setTime(date.getTime()+(days*24*60*60*1000));
    var expires = "; expires="+date.toGMTString();
    }
    else var expires = "";document.cookie = name+"="+value+expires+"; path=/";
    }
    function readCookie(name){
    var nameEQ = name + "=";var ca = document.cookie.split(';');
    for(var i=0;i < ca.length;i++){
    var c = ca[i];while (c.charAt(0)==' ') c = c.substring(1,c.length);
    if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);}
    return null;
    }
    window.onload = function(){redirect();createCookie('doRedirect','true','1');}
</script>

(the JS snippet used here was taken from Stack Overflow: JS to redirect to a splash page on first visit)

Thanks.