Function to obtain a URL parameter passed by GET from JavaScript. If the parameter does not exist returns null.

// Helper function to get parameters from the query string.
function getUrlParam(paramName) {
    var reParam = new RegExp('(?:[\?&]|&)' + paramName + '=([^&]+)', 'i');
    var match = window.location.search.match(reParam);
    return (match && match.length > 1) ? match[1] : null;
}