function minWindow(s, reqChars) {

​

    // hash to keep account of how many required characters we've checked off

    // each key will represent a character in reqChars

    // we preset each to 1 and will look to lower it to 0 when each is fulfilled

​

    const hash = {};

    for (let c of reqChars) {

        hash[c] = 1;

    };

​

    // trackers that we need

    // this is a counter to measure string length against

​

    let counter = reqChars.length;

    let begin = 0;

    let end = 0;

    let substrSize = s.length + 1;

    let head = 0;

​

    while (end < s.length) {

​

        // continue running while there's more elements in `s` to process

        if (s[end] in hash) {  // we've found a letter we needed to fulfill

​

            // modify the length counter, we can use this as part of our substring

            if (hash[s[end]] > 0) {

                counter -= 1;

            }