array intersection

array_intersection.js

// array intersection that correctly handles also duplicates

const intersection = (a1, a2) => {
  const cnt = new Map();
  a2.map(el => cnt.set(el, cnt.has(el) ? cnt.get(el) + 1 : 1));
  return a1.filter(el => cnt.has(el)).filter(el => {
    const left = cnt.get(el);
    if (0 < left) {
      cnt.set(el, left - 1);
      return true;
    }
    return false;
  });
};

intersection('1234'.split``, '3456'.split``); // [ '3', '4' ]
intersection('12344'.split``, '3456'.split``); // [ '3', '4' ]
intersection('1234'.split``, '33456'.split``); // [ '3', '4' ]
intersection('12334'.split``, '33456'.split``); // [ '3', '3', '4' ]