The library is ramdajs.com and it can be grabbed from npm.
I do not agree that everything should be handled with standard libraries. It's better for these things to be modular and versioned, instead of them being pulled from a big bloated standard library.
The code clearly does what was being asked in a very simple and non-broken way.
Perhaps the issue here is that non-JavaScript engineers are used to kitchensink standard libraries and avoid mixing and matching to get the best experience the ecosystem offers.
You're building a straw-man there. A standard library can certainly be made to be modular and versioned. It doesn't have to be bloated for any reason whatsoever. This is Javascript - you can chop up your scripts however you want.
The problem with mixing and matching libs is that no two JS devs do it the same. Some will even re-build the same lib that has already been built by hundreds of JS devs because NIH.
JavaScript is a very different language than the one people used to use 5 years back.