Querystring parsing

const paragraph = 'foo=1&foo=2&foo=3&blah=a&blah=b';
const regex = /([^=&]+)=([^&]*)/g
const array = [...paragraph.matchAll(regex)]

const obj = array.reduce((q,[kv, key, value]) => {
  q[key] = (q[key] ? q[key] + `,`: ``) + value
  return q
}, {});

const result = Object.keys(obj).map(a => `${a}=${obj[a]}`).join(`&`)
console.log(result)
// expected output: "foo=1,2,3&blah=a,b"