I built a view engine that relied on V8 for expression evaluation and flow. Not very stable of course, since it used RegEx, but it worked fine for what it was designed for.
The crown feature was the ability to pass in lazy-evaluated huge objects to that view model, so that the view model decided what was going to be used in order to display the view. Made it really flexible, while not sacrificing speed.

I was brainstorming for 2 days about the lazy loading part, and the gymnastics that had to be implemented for this to work.

After I wrote my final line of code and thought that this is it, I launched it, and it FUCKING WORKED! First try!
I was hyperventilating, walking around the apartment like crazy, doing random push-ups just to try to utilize some energy that I felt was fighting to burst out like a xenomorph out of the chest.

... 2 weeks later I found bugs. Had to re-learn how I did it. It's true what they say: if it was hard to write, it's even harder to debug. Fixed it eventually, but that part's not that exciting.