Functions like unionBy, intersectionBy, differenceBy, groupBy, mergeWith, mapValues, flatten already exist in JavaScript? Not even listing all the basic functio

Functions like unionBy, intersectionBy, differenceBy, groupBy, mergeWith, mapValues, flatten already exist in JavaScript? Not even listing all the basic functions not available before ES5/ES6 (find, filter, some, every, Object.keys, Object.values…). This lib is popular because nobody wants to rewrite these functions manually.