Track outbound links using google analytics. This script pushes an event when clicking on <a> tags with the `js-ext` class.

outbound.js

/**
 * Utility to wrap the different behaviors between W3C-compliant browsers
 * and IE when adding event handlers.
 *
 * @param {Object}     elem     The element on which to attache the event listener
 * @param {string}     event    The event type to listen for (e.g. load, click, etc.)
 * @param {function()} callback The callback function that receives the notification
 */
function addListener(elem, event, callback) {
    // W3C
    if (elem.addEventListener) {
        elem.addEventListener(event, callback);

        return true;
    }

    // IE
    else if (elem.attachEvent) {
        return elem.attachEvent('on' + event, callback);
    }

    return false;
}

/**
 * Track events on all links with the js-ext class
 */
document.addEventListener('DOMContentLoaded', function() {
    [].forEach.call(document.querySelectorAll(".js-ext"), function(el) {

        addListener(el, "click", function(event) {
            var title = el.dataset.title,
                link  = el.href,
                label;

            // If data-title is empty, use the href as the event label
            label = (title === undefined) ? link : title;

            console.log('Click on ' + label);

            try {
                // Old Google Analytics
                if (typeof _gaq !== "undefined") {
                    _gaq.push(['_trackEvent', 'Outbound Links', 'click', label]);
                }

                // New Google Analytics
                if (typeof ga !== "undefined") {
                    ga('send', 'event', 'Outbound Links', 'click', label);
                }
            } catch(err) {
                console.log(err);
            }

            // Delay by .5s if link target is not _blank, so that GA can track the event
            if (!el.target || el.target.match(/^_(self|parent|top)$/i)) {
                setTimeout(function() {
                    document.location.href = link;
                }.bind(el), 500);

                event.preventDefault ? event.preventDefault() : event.returnValue = !1;
            }
        });
    });
});

Instead of binding the event to all links with the js-ext class (on line #29), we can also use the following selector to target all links whose href starts with http and doesn't contain your domain name

document.querySelectorAll("a[href^='http']:not([href*='yourdomain.com'])")