fix: Allow serializers to operate on derived UrlSearchParams classes (

#647)

* Create a copy of `UrlSearchParams` when used as the base for a serializer

Follow-up to #646

* Update packages/nuqs/src/serializer.ts

Co-authored-by: François Best <github@francoisbest.com>

* test: Add test to show that existing params are not mutated

* chore: Fix formatting

---------

Co-authored-by: François Best <github@francoisbest.com>

packages/nuqs/src/serializer.test.ts

@@ -51,6 +51,13 @@ describe('serializer', () => {
     const result = serialize(search, { str: 'foo' })
     expect(result).toBe('?bar=egg&str=foo')
   })
+  test('Does not mutate existing params with URLSearchParams base', () => {
+    const serialize = createSerializer(parsers)
+    const searchBefore = new URLSearchParams('?str=foo')
+    const result = serialize(searchBefore, { str: 'bar' })
+    expect(result).toBe('?str=bar')
+    expect(searchBefore.get('str')).toBe('foo')
+  })
   test('with URL base', () => {
     const serialize = createSerializer(parsers)
     const url = new URL('https://example.com/path')

packages/nuqs/src/serializer.ts

@@ -70,8 +70,11 @@ function splitBase(base: Base) {
     const [path = '', search] = base.split('?')
     return [path, new URLSearchParams(search)] as const
   } else if (base instanceof URLSearchParams) {
-    return ['', base] as const
+    return ['', new URLSearchParams(base)] as const // Operate on a copy of URLSearchParams, as derived classes may restrict its allowed methods
   } else {
-    return [base.origin + base.pathname, base.searchParams] as const
+    return [
+      base.origin + base.pathname,
+      new URLSearchParams(base.searchParams)
+    ] as const
   }
 }