Mutations in GraphQL allow clients to modify data on the server, similar to how POST, PUT, and DELETE methods work in REST APIs. In this chapter, we will learn how to define and use mutations in GraphQL.

Defining Mutations in the Schema

In GraphQL, mutations are defined as part of the Mutation type. Mutations allow performing write operations such as creating, updating, or deleting records.

Schema Example with Mutations

Using our library API as an example:

graphql

In this schema, we define three mutations:

• createBook: To create a new book.
• updateBook: To update an existing book.
• deleteBook: To delete a book.

Implementing Resolvers for Mutations

Each mutation in the schema must have a corresponding resolver that handles the logic of the write operation.

Example Implementation of Resolvers

In our resolvers.js file, we would implement the resolvers for the mutations as follows:

javascript

Performing Mutations from the Client

Once mutations and resolvers are defined, clients can invoke mutations to modify the data on the server. GraphiQL again facilitates this process.

Example of Mutations in GraphiQL

• Mutation to create a new book:

graphql

• Mutation to update an existing book:

graphql

• Mutation to delete a book:

graphql

Error Handling

It is important to handle errors properly in the mutation resolvers to provide useful feedback to clients. You can throw custom errors using throw in JavaScript.

Example of Error Handling

javascript

Summary

In this chapter, we have learned to:

• Define mutations in the GraphQL schema.
• Implement resolvers to handle the mutations.
• Perform mutations from the client using GraphiQL.
• Handle errors in mutation resolvers.

With these skills, you can create GraphQL APIs that not only read data but also modify it safely and efficiently. In the next chapter, we will explore the use of resolvers in GraphQL in more detail.

[Placeholder: Diagram showing how mutation resolvers connect to the schema and modify data in response to client requests]