default params as a form of "let binding"

index.js

/* I've been thinking about default params a bit. And how you can use earlier params as values in the default params. 
It's a nice way to write functions without introducing any variable declaration statements in the body */ 

const greaterThan = num1 => num2 => num1 < num2;
const not = fn => (...args) => !fn(...args);
const quickSort = (
  nums,
  pivot = nums[Math.floor(Math.random() * nums.length)],
  left = nums.filter(not(greaterThan(pivot))),
  right = nums.filter(greaterThan(pivot)),
  done = new Set(nums).size <= 1
) => (done ? nums : [...quickSort(left), ...quickSort(right)]);

const filterWith = fn => (
  [first, ...rest],
  acc = [],
  passes = fn(first),
  next = passes ? acc.concat(first) : acc
) => (rest.length === 0 ? next : filterWith(fn)(rest, next));

const GCD = (
  num1,
  num2,
  larger = Math.max(num1, num2),
  smaller = Math.min(num2, num2),
  remainder = larger % smaller
) => (remainder === 0 ? smaller : GCD(larger, remainder));

const mapWith = fn => ([first, ...rest], acc = [], mapped = fn(first)) =>
  rest.length === 0 ? [...acc, mapped] : mapWith(fn)(rest, [...acc, mapped]);