Get URL Parameter

get-url-param.js

/**
 * Get URL param
 *
 * @param {string} param
 * @return {string|null}
 */
var getUrlParam = function (param) {

  // if input is invalid or there are no query params, return null
  if (typeof param !== 'string' || location.search === '') return null;

  // otherwise, split the search string on &
  var params = location.search.substring(1).split('&');

  // loop through the params and see if we have a match
  for (var i = 0; i < params.length; i++) {
    var keyValuePair = params[i].split('=');
    if (keyValuePair[0] === param) {
      return decodeURI(keyValuePair[1]);
    }
  }

  // if there's no match, return null
  return null;
  
};

Tested on latest versions of Chrome, Firefox, Safari, IE.