How do you test GraphQL APIs effectively?

Comprehensive GraphQL testing strategy: (1) Unit test resolvers: test resolver functions directly with mock context and args — no HTTP needed. expect(await userResolver({}, { id: '1' }, { db: mockDb })).toMatchObject({ name: 'Alice' });. (2) Integration test with a real schema: use graphql() function directly: const result = await graphql({ schema, source: GET_USER, variableValues: { id: '1' } });. (3) End-to-end tests: use supertest or a test HTTP client against a real running server with a test database. (4) Schema validation tests: test that schema loads without errors, required fields are non-null, deprecated fields have reasons. (5) Mock a GraphQL server: use graphql-tools addMocksToSchema for frontend testing without a real backend. (6) Contract testing: use Apollo Studio's schema checks in CI. (7) DataLoader testing: mock the batch function and verify batching behavior. Tools: Jest (test runner), graphql package (execution), @graphql-tools/mock, Apollo Server's testing utilities.