URLSearchParams parses any query-style string and exposes a Map-like API. Calling .get(name) returns the first occurrence as a string or null when the key is missing. Defaulting source to location.search lets the helper run anywhere a URL string can be passed (server-side requests, parsed URL objects, hash routes), which makes it easy to test without a real location. Note that .get always returns the first occurrence; for repeated keys (?tag=a&tag=b), use .getAll(name).

URL params are always strings, but call sites usually want numbers, booleans, or fall-throughs to a default. Inferring the target type from typeof fallback keeps the API tidy: callers pass the default they want and get back the same type. Guarding against NaN with Number.isFinite prevents ?page=oops from silently producing NaN in math downstream. Treating 'true' and '1' as truthy and everything else as falsy matches the convention used by most form-encoded URLs and feature-flag links.

URL plus URLSearchParams is the right combo for safe, encoding-aware query mutation: it handles already-encoded characters, repeated keys, and the leading ? correctly. Passing null/undefined to delete a key keeps the API symmetric with reactive form state where clearing a filter should drop the param entirely. In a real browser, swap the return for history.replaceState(null, '', url.search) so the URL updates without re-rendering the page or pushing a new history entry. Use pushState instead when the change should be back-button-navigable (route, page number).