refactor: update makeURLSearchParams to accept readonly non-Records (#8868)

packages/rest/__tests__/utils.test.ts

@@ -58,4 +58,31 @@ describe('makeURLSearchParams', () => {
 			expect([...params.entries()]).toEqual([['foo', 'bar']]);
 		});
 	});
+
+	describe('types', () => {
+		interface TestInput {
+			foo: string;
+		}
+
+		test("GIVEN object without index signature THEN TypeScript doesn't raise a type error", () => {
+			// Previously, `makeURLSearchParams` used `Record<string, unknown>` as an input, but that meant that it
+			// couldn't accept most interfaces, since they don't have an index signature. This test is to make sure
+			// non-Records can be used without casting.
+
+			const input = { foo: 'bar' } as TestInput;
+			const params = makeURLSearchParams(input);
+
+			expect([...params.entries()]).toEqual([['foo', 'bar']]);
+		});
+
+		test("GIVEN readonly object on a non-readonly generic type THEN TypeScript doesn't raise a type error", () => {
+			// While `Readonly<T>` type was always accepted in `makeURLSearchParams`, this test is to ensure that we can
+			// use the generic type and accept `Readonly<T>` rather than only [possibly] mutable `T`.
+
+			const input = Object.freeze({ foo: 'bar' } as TestInput);
+			const params = makeURLSearchParams<TestInput>(input);
+
+			expect([...params.entries()]).toEqual([['foo', 'bar']]);
+		});
+	});
 });