URLSearchParams.toString()

toString()

let url = new URL('https://example.com?foo=1&bar=2');
let params = new URLSearchParams(url.search);

//Add a second foo parameter.
params.append('foo', 4);
console.log(params.toString());
//Prints 'foo=1&bar=2&foo=4'

// note: params can also be directly created
let url = new URL('https://example.com?foo=1&bar=2');
let params = url.searchParams;

// or even simpler
let params = new URLSearchParams('foo=1&bar=2');