Control the order of the script loads

loadScript.js

var loadScript = (src, callback) => {
    var script = document.createElement('script');
    script.src = src;
    document.getElementsByTagName('head')[0].appendChild(script);
    script.onload = callback;
}

(function() {
    loadScript('firstScript.js', function() {
        console.log('First script loaded!');
        loadScript('secondScript.js', function() {
            console.log('Second script loaded!');
        });
    });
})();