My best experience was going from static HTML and non-preprocessed CSS to having my mind blown by Sass & learning JavaScript and what "API" even means (and starting on ruby and basics in command line). I actually feel like I'm a developer in some sense of the word.

That was a ton of growth in a year where I transitioned from a purely graphic design role to having an influence over development processes and rolled out a number of projects to production that I spearheaded.