I've used "one function" libs before, and i'll continue to. The most recent example is a small lib that added Array.prototype.reduce to browsers that don't have it. It's 31 lines total (un-compressed, un-minified, un-anything)

Installing that small, well tested, and standards compliant function using a single npm command is significantly better than trying to do it correctly myself (which would most likely take an hour or so just to look over it's exact functional requirements, and it would most likely have subtle bugs), half-ass it and write my own "not quite reduce but close-ish" that i now need to use everywhere and need to train new hires on because "I didn't want to install a 31 line lib, so now you have this non-standard system that's buggy in it's own strange ways", or copy-and-paste the code from somewhere muddying the legal status of my codebase.