This article discusses building a secure GraphQL API with Node.js using Fastify and Mercurius. The author creates a basic server, sets up a schema, resolvers, and implements authentication and authorization using the Mercurius Auth plugin. The API is secured by defining custom auth directives on fields in the schema, applying custom auth policies to these protected fields, and masking errors. The article also covers strategies for securing GraphQL APIs, including query depth limit, sanitizing and validating inputs, and more. The author tests the API using GraphQL Playground and demonstrates how to authenticate and authorize requests. Overall, this article provides a comprehensive guide on building a secure GraphQL API with Node.js using Fastify and Mercurius.