lodashA modern JavaScript utility library delivering modularity, performance, & extras.
This is, in my opinion, the missing standard library for JavaScript. It has practically every utility function I could conceive of needing and then some, all with comprehensive tests and excellent performance. The fp module is also a treat to use. I used to be of the mind that it was made obsolete by ES6, but having used it in a large work application I think it's an essential library for any serious JS project.