URLSearchParams()

new URLSearchParams(init);

// Retrieve params via url.search, passed into ctor
var url = new URL('https://example.com?foo=1&bar=2');
var params = new URLSearchParams(url.search);

// Pass in a string literal
var params2 = new URLSearchParams("foo=1&bar=2");
var params2a = new URLSearchParams("?foo=1&bar=2");

// Pass in a sequence of pairs
var params3 = new URLSearchParams([["foo", "1"], ["bar", "2"]]);

// Pass in a record
var params4 = new URLSearchParams({"foo": "1", "bar": "2"});

const url = new URL('https://example.com/?a=hello&b=world');

console.log(url.href);
// https://example.com/?a=hello&b=world

console.log(url.origin);
// https://example.com

const add_params = {
    c: 'a',
    d: new String(2),
    e: false.toString(),
}

const new_params = new URLSearchParams([
    ...Array.from(url.searchParams.entries()), // [["a","hello"],["b","world"]]
    ...Object.entries(add_params), // [["c","a"],["d","2"],["e","false"]]
]).toString();
console.log(new_params);
// a=hello&b=world&c=a&d=2&e=false

const new_url = new URL(`${url.origin}${url.pathname}?${new_params}`);

console.log(new_url.href);
// https://example.com/?a=hello&b=world&c=a&d=2&e=false

// Here it is as a function that accepts (URL, Record<string, string>)
const addSearchParams = (url, params = {}) =>
  new URL(
    `${url.origin}${url.pathname}?${new URLSearchParams([
      ...Array.from(url.searchParams.entries()),
      ...Object.entries(params),
    ]).toString()}`
  );