Prepend all anchors with preserved GET parameters

PreserveGetParamaters.js

/**************
* Prepend all anchors with preserved GET parameters
**************/
( function() {

	var domainsToDecorate = [
			'domain1.com', // add or remove domains (without https or trailing slash)
			'domain2.net'
		],
		queryParams = [
			'utm_medium', //add or remove query parameters you want to transfer
			'utm_source',
			'utm_campaign',
			'something_else'
		]
		
	// do not edit anything below this line
	var links = document.querySelectorAll( 'a' );
	
	// check if links contain domain from the domainsToDecorate array and then decorates
	for (var linkIndex = 0; linkIndex < links.length; linkIndex++) {
		for (var domainIndex = 0; domainIndex < domainsToDecorate.length; domainIndex++) {
			if (links[linkIndex].href.indexOf(domainsToDecorate[domainIndex]) > -1 && links[linkIndex].href.indexOf("#") === -1) {
				links[linkIndex].href = decorateUrl(links[linkIndex].href);
			}
		}
	}
	
	// decorates the URL with query params
	function decorateUrl(urlToDecorate) {
		urlToDecorate = (urlToDecorate.indexOf('?') === -1) ? urlToDecorate + '?' : urlToDecorate + '&';
		var collectedQueryParams = [];
		for (var queryIndex = 0; queryIndex < queryParams.length; queryIndex++) {
			if (getQueryParam(queryParams[queryIndex])) {
				collectedQueryParams.push(queryParams[queryIndex] + '=' + getQueryParam(queryParams[queryIndex]))
			}
		}
		return urlToDecorate + collectedQueryParams.join('&');
	}
	
	// borrowed from https://stackoverflow.com/questions/831030/
	// a function that retrieves the value of a query parameter
	function getQueryParam(name) {
		if (name = (new RegExp('[?&]' + encodeURIComponent(name) + '=([^&]*)')).exec(window.location.search)) return decodeURIComponent(name[1]);
	}
	
} )();