Fetch Inject was conceptualized and developed for After Dark to make it possible to deep link to full-sized Image Gallery images without blocking page load while working on the Hall of Mirrors add-on module.
The original implementation was 33 lines of code and I quipped each line took me an hour to write because I worked on the library for three days straight.
What I ended up with was something far more powerful than expected, with all kinds of use cases and a suitable replacement for far more complex libraries – sometimes with hundreds or even thousands of lines of code.
I slipped Fetch Inject into WordPress for a talk at WordCamp 2017 and measured a 200-300% improvement in page load performance. So I wrapped it into a single PHP file and made it a WordPress plug-in.