My solution to DonTheDeveloper JavaScript developer interview @ https://www.youtube.com/watch?v=7CWM-jLowpk

query-strings-transforms.js

const qs = "?foo=bar&zee=zoo&baz&foo=bar2&foo";

function getQueryObject(qs) {
  const withoutQueryDelimiter = qs.slice(1);
  const kvPairs = withoutQueryDelimiter.split("&");
  const output = {};

  for (const pair of kvPairs) {
    const [key, val] = pair.split("=");
    if (!key) continue;

    const paramVal = val === undefined ? "true" : val;

    if (Array.isArray(output[key])) {
      output[key].push(paramVal);
      continue;
    }

    if (output[key]) {
      output[key] = [output[key], paramVal];
      continue;
    }

    output[key] = paramVal;
  }
  return output;
}

// console.log(getQueryObject(qs))

function getQueryString(queryObj) {
  let str = "";
  const arrayValues = {};

  for (const key in queryObj) {
    let val;
    if (Object.hasOwnProperty.call(queryObj, key)) {
      val = queryObj[key];
    }

    if (!Array.isArray(val) && val === "true") {
      str = str.length == 0 ? (str += `${key}`) : (str += `&${key}`);
      continue;
    }

    if (!Array.isArray(val)) {
      str =
        str.length == 0 ? (str += `${key}=${val}`) : (str += `&${key}=${val}`);
      continue;
    }

    arrayValues[key] = val;
  }

  for (const key in arrayValues) {
    if (Object.hasOwnProperty.call(arrayValues, key)) {
      const arr = arrayValues[key];
      arr.forEach((val) => {
        if (val === "true") {
          str = !str.length
            ? (str += `${key}`)
            : (str += `&${key}`);
          return;
        }

        str = !str.length
          ? (str += `${key}=${val}`)
          : (str += `&${key}=${val}`);
        return;
      });
    }
  }

  return "?" + str;
}

// console.log(
//   getQueryString({ foo: ["bar", "bar2", "true"], zee: "zoo", baz: "true" })
// );