1. ES2015

window.location.search.substring(1).split('&')
  .reduce((result, query) => {
    const [k, v] = query.split('=');
    result[k] = decodeURI(v);
    return result;
  }, {});

2. ES2015 (Object.assignを使ったバージョン)

window.location.search.substring(1).split('&')
  .map(query => query.split('='))
  .reduce((result, [k, v]) => Object.assign(result, { [k]: decodeURI(v) }), {});

3. ES5

window.location.search.substring(1).split('&')
  .reduce(function(result, query) {
    var pair = query.split('=');
    result[pair[0]] = decodeURI(pair[1]);
    return result;
  }, {});

使い方

// ES2015
const getUrlParams = () => window.location.search.substring(1).split('&').reduce((result, query) => { const [k, v] = query.split('='); result[k] = decodeURI(v); return result; }, {});

//ES5
var getUrlParams = function() {
  return window.location.search.substring(1).split('&').reduce(function(result, query) { var pair = query.split('='); result[pair[0]] = decodeURI(pair[1]); return result; }, {});
}

getUrlParams();

実行結果

サンプルURL

https://www.google.co.jp/search?site=&source=hp&q=%E3%81%82%E3%81%84%E3%81%86+%E3%81%88%E3%81%8A&oq=%E3%81%82%E3%81%84%E3%81%86+%E3%81%88%E3%81%8A

結果

{ 
  site: "",
  source: "hp",
  q: "あいう+えお",
  oq: "あいう+えお"
}