countUnique.js

var unique = 'ab';
var all = 'Aaabbb';

var uniqueArr = unique.split('');
var allArr = all.split('');

function converToObj(accum, currentEl) {
	if (!accum[currentEl]) accum[currentEl] = true;

  return accum;
};
var uniqueObj = uniqueArr.reduce(converToObj, {});

function countThisShit(sum, currentEl) {
	if (uniqueObj[currentEl]) sum += 1;

  return sum;
};

allArr.reduce(countThisShit, 0)