index.js

let count = (s) => {
  return Array(s.length)
    .fill(0)
    .reduce((accu, _, i) => {
      const c = s[i];
      if (!accu[c]) {
        accu[c] = 1;
      } else {
        accu[c] += 1;
      }
      return accu;
    }, {});
};

function canShift(cs, ct, c) {
  let c1 = cs[c] || 0;
  let c2 = ct[c] || 0;
  if (c2 === 0) {
    return true;
  } else {
    return c1 > c2;
  }
}

function subCharCount(obj, c) {
  let o = Object.assign({}, obj);
  o[c] = o[c] - 1;
  return o;
}

function minWindowForSubString(s, cs, ct, start, end) {
  // let s1 = s.substr(start, end - start + 1);
  let minL = null;
  if (canShift(cs, ct, s[start])) {
    minL = minWindowForSubString(s, subCharCount(cs, s[start]), ct, start + 1, end);
  }

  let minR = null;
  if (canShift(cs, ct, s[end])) {
    minR = minWindowForSubString(s, subCharCount(cs, s[end]), ct, start, end - 1);
  }

  let cur = [start, end];
  let r = [cur, minL, minR].filter(t => !!t);
  r.sort((l, r) => {
    return (l[1] - l[0]) - (r[1] - r[0]);
  });
  return r && r[0];
}

let minWindow = (s, t) => {
  let cs = count(s);
  let ct = count(t);

  let start = 0;
  let end = s.length - 1;

  if (!Object.keys(ct)
    .every(key =>
      ct[key] <= (cs[key] || 0)
    )) {
    return '';
  }

  let range = minWindowForSubString(s, cs, ct, start, end);
  if (!range) {
    return '';
  } else {
    return s.substr(range[0], range[1] - range[0] + 1);
  }
};

// let r = minWindow('cabwefgewcwaefgcf', 'cae');
// let r = minWindow('a', 'aa');
// let r = minWindow('ab', 'a');
// let r = minWindow('ADOBECODEBANC', 'ABC');
console.log('r: ' + JSON.stringify(r, null, 4) + '\n');

// let k = count('cabwefgewcwaefgcf');
// console.log('k: ' + JSON.stringify(k, null, 4) + '\n');