Load a script via JavaScript and run a callback when its loaded

loadBackupScript.js

function loadBackupScript(callback){
    if (typeof callback !== 'function') {
       throw 'Not a valid callback';   
    }
    var script = document.createElement('script');
    script.src = 'http://www.geoplugin.net/javascript.gp';
    script.type = 'text/javascript'; 
    script.onload = callback;   
    var head = document.getElementsByTagName("head")[0];
    head.appendChild(script);
}

// Usage:
loadBackupScript(function() { alert('loaded'); });