UTM link decorator, AMD module. It decorates all your links you pass with the current UTM parameters from URL.

usage.js

require( [ 'jquery', 'utm-decorator' ] , function ( $, Decorator ) {
	'use strict';

	// decorate all links to themeforest and to our demo page on page load
	$( function() {
		var utmDecorator = new Decorator;
		$( '[href*="themeforest.net"], [href*="proteusthemes.com"]' ).each( function ( index, $el ) {
			utmDecorator.decorate( $el );
		} );
	} );

} );

utm-decorator.js

define( [ 'jquery' ], function ( $ ) {
	/**
	 * Constructor
	 * @return {[type]} [description]
	 */
	var utmDecorator = function () {
		this.autosetParameters();
		this.stringifyObj( this.parametersObj );
	};

	$.extend( utmDecorator.prototype, {
		// from: https://support.google.com/analytics/answer/1033867?hl=en
		utmParams: [ 'utm_source', 'utm_medium', 'utm_term', 'utm_content', 'utm_campaign' ],

		parametersObj: {},

		urlAppend: '',

		/**
		 * Helper function for getting parameter by name
		 * @see  http://stackoverflow.com/a/901144
		 * @param  {string} name
		 * @return {string}
		 */
		getParameterByName: function (name) {
			name = name.replace(/[\[]/, "\\[").replace(/[\]]/, "\\]");
			var regex = new RegExp("[\\?&]" + name + "=([^&#]*)"),
			results = regex.exec(location.search);
			return results === null ? "" : decodeURIComponent(results[1].replace(/\+/g, " "));
		},

		/**
		 * Pass the DOM element and it will output the decorated link
		 * @param  {DOM} el
		 */
		decorate: function ( el ) {
			var $el = $( el );

			var prepend = $el[0].search.length > 0 ? '&' : '?';

			$el.attr( 'href', $el.attr( 'href' ) + prepend + this.urlAppend );

			return this;
		},

		/**
		 * Set parameters, based on the existing page
		 * @return this
		 */
		autosetParameters: function () {
			// reset
			this.parametersObj = {};

			// check every
			$.each( this.utmParams, $.proxy( function ( index, utmParam ) {
				var utmParamVal = this.getParameterByName( utmParam );
				if ( utmParamVal ) {
					this.parametersObj[ utmParam ] = utmParamVal;
				}
			}, this ) );

			return this;
		},

		/**
		 * Build the HTTP GET query
		 * @param  {object} obj
		 * @return this
		 */
		stringifyObj: function ( obj ) {
			var urlAppend = [];
			$.each( obj, $.proxy( function ( key, val ) {
				urlAppend.push( key + '=' + val );
			}, this ) );

			this.urlAppend = urlAppend.join( '&' );

			return this;
		}
	} );

	return utmDecorator;
} );