How to Fix Common GraphQL Errors

In this tutorial, you'll learn about How to Fix Common GraphQL Errors. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.

The Problem

Your GraphQL client receives:

{
  "errors": [
    {
      "message": "Cannot query field \"email\" on type \"User\".",
      "locations": [{"line": 2, "column": 3}]
    }
  ]
}

Or any other GraphQL error in the errors array. GraphQL returns errors alongside partial data instead of HTTP error codes.

Quick Fix

1. Fix "Cannot query field" errors

The field does not exist in the schema:

# Wrong — 'email' is not in the schema for this query
query {
  users {
    id
    name
    email  # field does not exist
  }
}

# Right — check the schema for available fields
query {
  users {
    id
    name
    emailAddress  # actual field name
  }
}

2. Fix "Field X is not defined" in mutations

Mutation arguments must match the schema:

# Wrong — missing required argument
mutation {
  createUser(name: "John")  # 'email' is required
}

# Right
mutation {
  createUser(name: "John", email: "john@example.com")
}

3. Handle resolver errors

Resolvers that throw errors return null with an error message:

// Wrong — throwing generic error
const resolvers = {
  Query: {
    user: async (_, { id }) => {
      const user = await db.findUser(id)
      if (!user) throw new Error('Not found')
      return user
    }
  }
}

// Right — use GraphQLError for better messages
const { GraphQLError } = require('graphql')

const resolvers = {
  Query: {
    user: async (_, { id }) => {
      const user = await db.findUser(id)
      if (!user) throw new GraphQLError('User not found', {
        extensions: { code: 'NOT_FOUND', http: { status: 404 } }
      })
      return user
    }
  }
}

4. Fix validation errors

Check the query against the schema for type mismatches:

# Wrong — passing String instead of Int
query {
  user(id: "abc") { name }
}

# Right
query {
  user(id: 123) { name }
}

5. Debug network errors

Check the HTTP status and response:

# GraphQL always returns 200, so check the response body
curl -v -X POST https://api.example.com/graphql \
  -H "Content-Type: application/json" \
  -d '{"query":"{ users { id name } }"}'

Prevention

• Use the GraphQL schema as the source of truth for all queries.
• Set up schema introspection in development tools like GraphiQL.
• Validate queries against the schema before deploying.
• Log all GraphQL errors with the query and variables.
• Use Apollo Studio or similar tools to monitor error rates.

Common Mistakes with GraphQL error

1. Using head and tail instead of pattern matching, causing runtime errors on empty lists
2. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks
3. Using return to exit a function early instead of wrapping a pure value in the monad

These mistakes appear frequently in real-world API code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

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