The URL and URLSearchParams classes help you work with URLs cleanly.
const url = new URL("https://example.com/path?a=1");
console.log(url.pathname); // "/path"
const params = new URLSearchParams(url.search);
console.log(params.get("a")); // "1"
params.set("b", "2");
console.log(params.toString()); // "a=1&b=2"