URL.searchParams

The searchParams readonly property of the URL interface returns a URLSearchParams object allowing access to the GET decoded query arguments contained in the URL.

A URLSearchParams object.

If the URL of your page is https://example.com/?name=Jonathan%20Smith&age=18 you could parse out the name and age parameters using:

let params = (new URL(document.location)).searchParams;
let name = params.get('name'); // is the string "Jonathan Smith".
let age = parseInt(params.get('age')); // is the number 18