Scripts At The Bottom Is Not Enough

You may remember one of the original webperf rules was to put scripts at the bottom.

Over the last few years browsers have been adjusting, now it is common for JavaScript to have higher loading priority via a pre-loader. This can result in scripts at the bottom effectively loading the same as scripts at the top.

Patrick Meenan on how it is supposed to work ( emphasis mine ):

– The preload scanner scans the whole document as it comes in and issues fetch requests.
– The main parser sends a signal when it reaches the start of the body and sends fetch requests for any resources it discoveres that are not already queued.
– The layout engine increases the priority of in-viewport visibile images when it does layout.
– The resource loader treats all script and css as critical, regardless of where it is discovered (in the head or at the end of the body). see crbug.com/317785
– Resource loader loads in 2 phases. The first (critical phase) delays non-critical resources except for 1 image at a time. Once the main parser reaches the body tag it removes the constraint and fetches everything.

At this point you’ll likely need to use async/defer to really push JavaScript loading out of the critical category. Of course that isn’t a slam dunk either. Paul Irish notes this limitation with IE:

TL;DR: don’t use defer for external scripts that can depend on eachother if you need IE

Even after 20 years of development there are days where browser environments still feel like the wild west.