@tomzer0/collections/index.js

Array collection functions

const identity = i => i;
const last = arr => arr[arr.length-1];
const also = (it, fn) => {
  fn(it);
  return it;
}

const flatten = arr => arr.reduce((acc, n) => {
  if (Array.isArray(n)) return acc.concat(flatten(n));
  return acc.concat(n);
}, []);

const associate = (arr, keySelector, valueSelector=identity, initial={}) => arr.reduce((acc, n) => 
  Object.assign({ [keySelector(n)]: valueSelector(n) }, acc),
initial);

const chunked = (arr, size, transform=identity) => arr.reduce((acc, n) => {
  if (last(acc).length < size) return also(acc, it => last(acc).push(n));
  return also(acc, it => {
    it.push(transform(it.pop()));
    it.push([n])
  });
}, [[]]);

const union = (...arrs) => arrs.reduce((res, n) => 
  [...new Set([...res, ...n])],
[]);

const distinct = arr => [...new Set(arr)];

const drop = (arr, amt) => arr.slice(amt);
const dropWhile = (arr, pred, curr=0) => pred(arr[curr]) ? dropWhile(arr.slice(1), pred, curr+1) : arr;

const take = (arr, amt) => arr.slice(0, amt);
const takeWhile = (arr, pred, curr=0, acc=[]) => pred(arr[curr]) ? takeWhile(arr, pred, curr+1, [...acc, arr[curr]]) : acc;

const group = (arr, keySelector) => Object.values(arr.reduce((acc, n) => {
  const key = keySelector(n);
  return also(acc, it => (it[key] = [...(it[key] || []), n]) );
}, {}));

const partition = (arr, pred=identity) => arr.reduce((acc, n) => 
  also(acc, it => it[+!!pred(n)].push(n)),
[ [], [] ]);

const zip = (a, b) => Array.from(Array(Math.max(a.length, b.length))).reduce((acc, _, i) =>
  also(acc, it => (it[i] = [a[i], b[i]])),
[]);

module.exports = {
  last,
  also,
  flatten,
  associate,
  chunked,
  union,
  distinct,
  drop,
  dropWhile,
  take,
  takeWhile,
  group,
  partition,
  zip,
};