Get Query String Parameters as JSON Object with JavaScript

script.js

function getUrlParameter(name, defaultValue = "") {
  query = query.substring(location.search.indexOf("?") + 1);

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

  var decode = function(str) {
    return decodeURIComponent(str.replace(decodeRE, " "));
  };

  var params = {},
    e;
  while ((e = re.exec(query))) {
    var 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;
  }

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

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

  return params.hasOwnProperty(name) ? params[name] : defaultValue;
}