DOM Enlightenment Book

HTML

<script src="https://getfirebug.com/firebug-lite-debug.js"></script>
<!-- defer, don't block just ignore this until the <html> element node is parsed -->
<script async src="http://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.3.3/underscore-min.js"></script>
<!-- defer, don't block just ignore this until the <html> element node is parsed -->
<script async src="http://cdnjs.cloudflare.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<!-- defer, don't block just ignore this until the <html> element node is parsed -->
<script async src="http://cdnjs.cloudflare.com/ajax/libs/jquery-mousewheel/3.0.6/jquery.mousewheel.min.js"></script>

<script>
//We know that jQuery is not available because this occurs before the closing html element
console.log(window['jQuery'] === undefined); //logs true
    
//Only after everything is loaded can we safely conclude that jQuery was loaded and parsed
document.body.onload = function(){console.log(jQuery().jquery)}
</script>