MinimumWindowSubstring.js

function minimumWindowSubstring(str, substr) {
  let lettersSeen = {};
  let lettersNeeded = {};
  let lettersMissing = 0;

  for(let i = 0; i < substr.length; i++) {
    if(substr[i] in lettersNeeded){
      lettersNeeded[substr[i]] += 1;
    } else {
      lettersNeeded[substr[i]] = 1;
      lettersSeen[substr[i]] = 0;
      lettersMissing += 1;
    }
  }

  let fast = 0;
  let slow = 0;

  let result = [-Infinity, Infinity];

  for(fast; fast < str.length; fast++) {

    if(str[fast] in lettersNeeded) {
      lettersSeen[str[fast]] += 1;
      if(lettersSeen[str[fast]] === lettersNeeded[str[fast]]) {
        lettersMissing -= 1;
      }
    }
    
    while(lettersMissing === 0) {
      if(fast - slow < result[1] - result[0]) {
        result[0] = slow
        result[1] = fast
      }
      if(str[slow] in lettersNeeded) {
        lettersSeen[str[slow]] -= 1;
        if(lettersSeen[str[slow]] < lettersNeeded[str[slow]]) {
          lettersMissing += 1;
        }
      }

      slow++;
    }
  }

  return result[0] === -Infinity ? "" : str.slice(result[0], result[1] + 1);
}

This is really helpful @spiterman :)