JavaScript is a non-blocking programming language, which means that one chunk of JS code will not necessarily wait for the lines above it to execute before beginning itself. In many cases this is a pretty awesome feature of the language. Imagine if your website was only able to run one window.onload call at a time. If your page needed to perform a number of calculations or data insertions in order to render correctly, things would take forever to load and users would see a bunch of unstyled content and blank boxes at the beginning of every page load. Yuck.