Vanilla JS groupBy implementation

groupBy.js

// utilities
const includes = (needle) => (haystack) => (haystack.indexOf(needle) > -1);
const includesAtPos = (needle) => (position) => (haystack) => (haystack.indexOf(needle) === position);
const testFor = (re) => (str) => re.test(str);
const and = (...fnList) => (data) => fnList.every((fn) => fn(data));

const groupBy = (fn) => (arr) => arr.reduce(
  (out, val) => {
    let by = fn(val);
    (out[by] = out[by] || []).push(val);
    return out;
  },
  {}
);

const tagAttrSorter = (str) => (
  includesAtPos('.')(0)(str) ? 'classes'
  : includesAtPos('#')(0)(str) ? 'ids'
  : and(includes('='), testFor(/^(?!on)/))(str) ? 'attrs'
  : 'remainder');

const groupByTagAttrs = groupBy(tagAttrSorter);

groupByTagAttrs([
  '.this',
  '.that',
  '#id1',
  '#id2',
  'data-key=val',
  'onclick=alert()'
]);