Day 3: Mutations, Input Types, and Validation

Today's Objective

Write data through GraphQL with mutations. Input types, custom scalars, server-side validation, and the error handling patterns that give clients actionable feedback.

Mutations in the Schema

Schema additions

const typeDefs = `#graphql type Mutation { createUser(input: CreateUserInput!): User! updateUser(id: ID!, input: UpdateUserInput!): User! deleteUser(id: ID!): Boolean! createPost(input: CreatePostInput!): Post! } input CreateUserInput { name: String! email: String! } input UpdateUserInput { name: String email: String } input CreatePostInput { title: String! body: String authorId: ID! } `

Mutation Resolvers

const resolvers = {
  Mutation: {
    createUser: (_, { input }) => {
      const user = { id: String(users.length + 1), ...input };
      users.push(user);
      return user;
    },
    updateUser: (_, { id, input }) => {
      const user = users.find(u => u.id === id);
      if (!user) throw new Error('User not found');
      Object.assign(user, input);
      return user;
    },
    deleteUser: (_, { id }) => {
      const idx = users.findIndex(u => u.id === id);
      if (idx === -1) return false;
      users.splice(idx, 1);
      return true;
    },
  }
};

Running a mutation

mutation CreateUser {
  createUser(input: {
    name: "Carol"
    email: "[email protected]"
  }) {
    id
    name
    email
  }
}

📝 Day 3 Exercise

Add Mutations to Your API

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Day 3 Summary

Mutations modify data. Same GraphQL format as queries but use the mutation keyword.
Input types group mutation arguments: input: CreateUserInput!.
Throw errors in resolvers for validation failures — GraphQL catches and returns them in the errors array.
The mutation response should include the created/updated object so the client can update its cache.

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Day 2: Queries: Fetching Exactly What You Need

Day 4: Authentication and Context

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What's Next

The foundations from today carry directly into Day 4. In the next session the focus shifts to Subscriptions and Real-Time Data — building directly on everything covered here.

Supporting Videos & Reading

Go deeper with these external references.

YouTube

Mutations, Input Types, and Validation — Video Walkthroughs Community tutorials and walkthroughs covering the concepts in this lesson.

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YouTube

Mutations, Input Types, and Validation Explained Deep-dive explanations and live-coding sessions from top educators.

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Official Docs

Official Documentation Primary reference documentation for the technologies covered in this lesson.

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GitHub

Open Source Examples Real-world codebases demonstrating the patterns taught in this lesson.

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Day 3 Checkpoint

Before moving on, verify you can answer these without looking:

What is the core concept introduced in this lesson, and why does it matter?
What are the two or three most common mistakes practitioners make with this topic?
Can you explain the key code pattern from this lesson to a colleague in plain language?
What would break first if you skipped the safeguards or best practices described here?
How does today's topic connect to what comes in Day 4?

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Continue To Day 4

Day 4: Authentication and Context

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