1. Introduction

In this tutorial, we will be diving into the world of error handling and validation in GraphQL. The goal is to help you understand how to manage errors during mutation operations and validate the data passing through your GraphQL server.

By the end of this tutorial, you will learn:
- The basics of error handling in GraphQL
- How to use error handling in mutations
- How to validate mutations

Prerequisites:
- Basic knowledge of GraphQL
- Familiarity with JavaScript

2. Step-by-Step Guide

Error Handling in GraphQL

In GraphQL, errors can be handled at two levels:
- At the GraphQL level: these errors are usually syntax errors or validation errors.
- At the application level: these errors are related to the application's business logic.

Validating Mutations

Mutations in GraphQL allow us to modify data. It's essential to validate these mutations to prevent invalid data from entering our system.

3. Code Examples

Handling Errors in Mutations

Here's an example of how you might handle errors in a GraphQL mutation:

const resolvers = {
 Mutation: {
   createPost: async (_, { input }, context) => {
     try {
       const newPost = await context.Post.create(input);
       return newPost;
     } catch (error) {
       throw new Error("Failed to create post");
     }
   },
 },
};

In this snippet, we are trying to create a new post. If there's an error during this operation, we catch it and throw a new error with a custom message: "Failed to create post".

Validating Mutations

When validating mutations, it's common to use a library like joi or yup. Here's an example using yup:

const yup = require("yup");

const schema = yup.object().shape({
 title: yup.string().required(),
 body: yup.string().required().max(500),
});

const resolvers = {
 Mutation: {
   createPost: async (_, { input }, context) => {
     try {
       await schema.validate(input);
       const newPost = await context.Post.create(input);
       return newPost;
     } catch (error) {
       throw new Error("Validation failed");
     }
   },
 },
};

In this example, we define a yup schema to validate our mutation input. If validation fails, we throw an error.

4. Summary

In this tutorial, you've learned about error handling and validation in GraphQL. You've seen how to manage errors during mutation operations and validate data before it's stored.

Next steps for learning would be to dive deeper into the various error handling and validation libraries available for GraphQL.

Here are some additional resources:
- Official GraphQL Documentation
- Yup Documentation

5. Practice Exercises

1. Write a mutation for updating a post. Handle any potential errors.
2. Validate the update post mutation using a yup schema.

Solutions and tips for these exercises will be posted in the next tutorial. Happy coding!