How to get a parameter's value from URL and set a client ID into GA tracker, to retrieve it later

find-parameter-clientid-from-ga.js

/**
 * Retrieve parameter from URL
 */
function findParameter(param) {
  var result = 0, tmp = [], BreakException = {};

  // Iterate on all and breaks when param is found
  // @see http://stackoverflow.com/a/2641374
  try {
    location.search
      .substr(1)
      .split('&')
      .forEach(function (item) {
        tmp = item.split('=');

        // Check parameter
        if (tmp[0] !== param) {
          return;
        }

        // Parameter found!
        result = decodeURIComponent(tmp[1]);
        throw BreakException;
      });
  } catch (e) {
    if (e !== BreakException) throw e;
  }

  // Return parameter's value
  return result;
}

// Build usefull vars
var _clientid = findParameter('_clientid'),
  _ua = 'UA-XXXXXXXX-Y',
  _obj = {'allowLinker': true};

// Check client ID
if (0 != _clientid) {
    _obj.clientId = _clientid;
}

// Initliaze GA script
(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
})(window,document,'script','https://www.google-analytics.com/analytics.js','ga');
ga('create', _ua, 'auto', _obj);
ga('send','pageview');

// Instanciate clientId from tracker
var clientId = '';
ga(function (tracker) {
  clientId = tracker.get('clientId');
});