Debugging Tools for JSON-based GraphQL Responses

Introduction

GraphQL APIs primarily communicate using JSON. A GraphQL request sends a query (or mutation/subscription) to a single endpoint, and the API responds with a JSON object containing the requested data, and potentially an array of errors or additional information in an extensions field. While this standardized format is powerful, debugging unexpected results, missing data, or errors within these JSON responses requires specific tools and techniques.

This article explores various tools and approaches developers can use to effectively debug JSON responses from GraphQL APIs, suitable for developers of all levels.

Basic Browser Developer Tools

The first line of defense for debugging any web request, including GraphQL, is your browser's built-in developer tools.

Network Tab

The Network tab is essential for inspecting the request and the raw response.

• Locate the Request: Find the POST request made to your GraphQL endpoint (often something like /graphql or /api). You might need to filter by "XHR" or "Fetch".
• Status Code: Check the HTTP status code. A 200 OK status doesn't necessarily mean the GraphQL operation succeeded; it just means the HTTP request to the server was successful. GraphQL errors are often returned with a 200 OK status code but included in the errors field of the JSON body. Other status codes (like 400, 500) indicate HTTP-level problems (bad request, server error) before the GraphQL layer was fully processed.
• Payload/Headers: Examine the request payload (the GraphQL query/mutation and variables) and headers (like Authorization).
• Response Tab: This is where you'll see the raw JSON response from the server. Most browsers provide a "Preview" or "Response" tab that formats the JSON for readability.
• Timing: The Timing tab shows how long the request took, which is useful for diagnosing performance issues.

{
  "data": {
    "user": {
      "id": "123",
      "name": "Alice",
      "email": "alice@example.com"
    }
  },
  "errors": null,
  "extensions": {
    "tracing": { /* ... timing details ... */ }
  }
}

Console Logging

Logging the response data directly in your client-side code is a straightforward way to inspect its structure and values at runtime.

fetch('/graphql', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    query: `
      query GetUser($userId: ID!) {
        user(id: $userId) {
          id
          name
          email
        }
      }
    `,
    variables: { userId: '123' },
  }),
})
.then(res => res.json())
.then(data => {
  console.log('GraphQL Response:', data);
  // Now inspect the 'data' or 'errors' properties of the parsed JSON
  if (data.errors) {
    console.error('GraphQL Errors:', data.errors);
  }
  if (data.data) {
    console.log('GraphQL Data:', data.data);
  }
})
.catch(error => {
  console.error('Fetch Error:', error);
});

Modern browser consoles allow you to expand and explore the nested structure of JSON objects logged this way.

GraphQL-Specific Tools & IDEs

Tools designed specifically for GraphQL provide enhanced features for exploring schemas, building queries, and, importantly, inspecting responses in a structured, user-friendly way.

GraphQL IDEs (GraphiQL, Apollo Sandbox, GraphQL Playground, Altair)

These are interactive environments that connect directly to your GraphQL endpoint.

• Query Execution: You can type and execute queries/mutations.
• Schema Exploration: They often provide schema documentation, making it easy to understand available types and fields.
• Structured Response View: The "Response" pane automatically formats the incoming JSON, often with syntax highlighting and collapsible sections, making it much easier to read than the raw text in a Network tab. You can clearly see thedata, errors, and extensions fields.

Using these IDEs is often the easiest way to isolate a specific query or mutation and debug its direct response without the complexity of the full client application.

Browser Extensions (e.g., Apollo DevTools)

Extensions like Apollo DevTools (for applications using Apollo Client) add a dedicated panel to your browser's developer tools.

• Operation View: They list the GraphQL operations performed by your application.
• Variable and Response Inspection: For each operation, you can inspect the variables used and the full JSON response received by the client library. This is particularly useful for debugging issues specific to how your client library handles or caches data.
• Cache Inspection: If using a client with a cache, you can often inspect the contents of the cache.

Understanding the GraphQL JSON Structure

A standard GraphQL JSON response object contains up to three root fields:

• data: This field contains the result of the GraphQL operation if it was successful (at least partially). Its shape mirrors the shape of the query. If any field in the query resolved to an error where the field was non-nullable in the schema, the data field itself might be null at the top level.
• errors: This optional field is an array of error objects. It is present if any error occurred during the execution of the GraphQL operation. These errors can range from syntax errors in the query to runtime errors resolving fields.
• extensions: This optional field is a map that can contain arbitrary data from the server, often used for non-standard information like tracing, caching hints, or custom error codes.

{
  "data": {
    "user": null
  },
  "errors": [
    {
      "message": "User not found",
      "locations": [ { "line": 2, "column": 3 } ],
      "path": [ "user" ],
      "extensions": {
        "code": "NOT_FOUND",
        "http": { "statusCode": 404 }
      }
    }
  ],
  "extensions": { /* ... */ }
}

Common Debugging Scenarios

Missing Data (null fields)

If you expect data in a field but receive null, check the following:

• errors Array: Is there an entry in the errors array corresponding to this field? If a field that is defined as Non-Nullable (using ! in the schema) fails to resolve, the error will "bubble up" and nullify the parent field, or even the entire data object if it's at the root.
• Resolver Logic: If the field is nullable,null might be the intended result from the server-side resolver logic (e.g., "user not found"). Check the backend logs or add logging in the resolver function.
• Permissions: Does the authenticated user have permission to access this data? The backend might return null or an error if not authorized.

Understanding the errors Array

The errors array provides crucial information.

• message: A human-readable description of the error.
• locations: An array of locations in the GraphQL query string where the error occurred (line and column number).
• path: An array of strings or indices describing the path in the JSON response data where the error happened (e.g., ["user", "address"]).
• extensions: Often contains custom error codes (e.g., "NOT_AUTHENTICATED", "VALIDATION_ERROR"), or other details added by the server. This is highly valuable for programmatic error handling and detailed debugging.

Always inspect the errors array first when something goes wrong with the data you receive.

Unexpected Data Types or Structures

If a field returns a type you didn't expect (e.g., a string instead of a number), verify the following:

• GraphQL Schema: Compare the response structure and types to the GraphQL schema definition. Is the server returning data that doesn't match the schema?
• Resolver Implementation: The backend resolver might be incorrectly formatting or transforming the data before returning it.

Analyzing Large Responses

Large responses can be difficult to navigate in browser developer tools alone.

• Save as JSON: In the Network tab, you can often right-click the response preview and save the response as a JSON file.
• External JSON Viewers/Formatters: Open the saved JSON file in a dedicated JSON viewer or online formatter. These tools provide better syntax highlighting, searching capabilities, collapsible sections, and sometimes validation, making it easier to find specific data points or understand the overall structure.
• Client-Side Processing: If the issue is in how your client-side code processes a large response, step through the code with a debugger while inspecting the data variable.

Tips for Efficient Debugging

• Start Simple: If a complex query fails, try fetching only a few basic fields first to see if that works. Gradually add more fields or complexity until the issue reappears.
• Isolate with an IDE: Once you suspect an issue with a specific query or mutation, run it directly in a GraphQL IDE to rule out client-side code issues.
• Check Backend Logs: Collaborate with backend developers (or check logs yourself if you have access) for server-side errors or warnings that correspond to your request.
• Use extensions: If your backend includes debugging information in the extensions field (like database query timings, cache hits/misses), learn to read and utilize this information.
• Schema Awareness: Have the GraphQL schema documentation handy. Understanding the expected types, nullability, and relationships is fundamental to diagnosing response issues.

Conclusion

Debugging JSON-based GraphQL responses involves a combination of general web debugging skills and GraphQL-specific tools. By understanding the standard GraphQL response structure (data, errors, extensions), effectively using browser developer tools, leveraging dedicated GraphQL IDEs and extensions, and applying systematic isolation techniques, you can quickly identify the root cause of issues whether they lie in the query itself, the client-side handling, or the backend resolvers.