Parse query string. (Just use URLSearchParams instead of this)

parse-query-string.js

function parseQueryString(query) {
  query = query.substring(query.indexOf('?') + 1);

  let re = /([^&=]+)=?([^&]*)/g;
  let decodeRE = /\+/g;

  let decode = function(str) {
    return decodeURIComponent(str.replace(decodeRE, ' '));
  };

  let params = {},
    e;
  while ((e = re.exec(query))) {
    let k = decode(e[1]),
      v = decode(e[2]);
    if (k.substring(k.length - 2) === '[]') {
      k = k.substring(0, k.length - 2);
      (params[k] || (params[k] = [])).push(v);
    } else params[k] = v;
  }

  let assign = function(obj, keyPath, value) {
    let lastKeyIndex = keyPath.length - 1;
    for (let i = 0; i < lastKeyIndex; ++i) {
      let key = keyPath[i];
      if (!(key in obj)) obj[key] = {};
      obj = obj[key];
    }
    obj[keyPath[lastKeyIndex]] = value;
  };

  for (let prop in params) {
    let structure = prop.split('[');
    if (structure.length > 1) {
      let levels = [];
      structure.forEach(function(item, i) {
        let key = item.replace(/[?[\]\\ ]/g, '');
        levels.push(key);
      });
      assign(params, levels, params[prop]);
      delete params[prop];
    }
  }
  return params;
}