Would it be shorter if it didn’t, though?

The expectation is that you import the whole library and not just one function. At which point, lodash is smaller than if each function was implemented separately.

Glimpsing at the source code - yes, definitely.

For example, the code imports "copyArray", a 9LoC function that does exactly the same as the built-in Array.slice() would do.

Same goes for other imported helpers like "arrayEach", which could be replaced by the built-in Array.every().

It's basically a bunch of unnecessary polyfills for built-ins.

Unnecessary _now_ perhaps, but not unnecessary when lodash was written. In 2009, when underscore.js came out (from which lodash was a fork) Array.every() was not something you could rely on. Chrome just came out a year earlier (and didn't support Array.every until version 4) and half the world was on IE 6-7-8