In a JavaScript GraphQL server, middleware plays a crucial role in intercepting and modifying requests and responses. This allows us to implement data transformations, such as sanitization, validation, and normalization, in a centralized manner.

In this blog post, we will explore how to leverage middleware to implement data transformations in a JavaScript GraphQL server. We will be using the popular Apollo Server library as an example.

What is Middleware in GraphQL?

In the context of GraphQL, middleware refers to functions that intercept GraphQL requests and responses. These functions are executed in a sequential order, allowing us to modify the data before it reaches the resolver functions or is sent back to the client.

Middleware functions receive the GraphQL resolve function, which allows them to continue the execution flow by calling the next middleware or the resolver function itself.

Implementing Data Transformations with Middleware

Let’s say we want to sanitize user input by removing any HTML tags before it reaches the resolver function. We can achieve this by creating a middleware function that sanitizes the request:

const sanitizeMiddleware = (resolve, parent, args, context, info) => {
  // Sanitize user input
  args.input = sanitize(args.input);

  // Continue execution
  return resolve();
};

In this example, sanitize is a function that removes HTML tags from the input. We then modify the args object to reflect the sanitized input before calling the resolve function to continue the execution flow.

Next, we will need to apply this middleware to our GraphQL server. In Apollo Server, middleware can be added using the applyMiddleware function provided by the apollo-server-express package:

const express = require('express');
const { ApolloServer } = require('apollo-server-express');
const app = express();

// ...Other server setup code...

const server = new ApolloServer({
  typeDefs,
  resolvers,
  context: // ...,
  plugins: [
    {
      requestDidStart: () => ({
        willResolveField: sanitizeMiddleware,
      }),
    },
  ],
});

server.applyMiddleware({ app });

// ...Start the server...

In the example above, we create an instance of ApolloServer and define our GraphQL schema (typeDefs) and resolvers (resolvers). We then add our middleware function to the plugins option using the requestDidStart hook and the willResolveField event.

Finally, we call the applyMiddleware function to integrate the Apollo Server with our Express server.

Conclusion

Using middleware in a JavaScript GraphQL server allows us to implement data transformations, such as sanitization, validation, and normalization, in a centralized and reusable manner. By intercepting requests and responses, we can modify the data before it reaches the resolver functions or is sent back to the client.

In this blog post, we explored how to implement data transformations with middleware, specifically in the context of a JavaScript GraphQL server using Apollo Server. By leveraging middleware, we can improve the maintainability and reusability of our server code.

#GraphQL #JavaScript