Solving the “Next-Swagger-Doc Error: Cannot convert undefined or null to object” Conundrum

If you’re reading this, chances are you’ve stumbled upon the infamous “Next-Swagger-Doc Error: Cannot convert undefined or null to object” error while trying to set up your Next.js project with Swagger documentation. Fear not, dear developer, for you’re about to embark on a journey to conquer this pesky issue and emerge victorious with a beautifully documented API.

What’s behind the error?

The error message “Cannot convert undefined or null to object” is a cryptic message that can be misleading, making it challenging to identify the root cause. However, don’t worry; we’ll break it down together.

When Next.js attempts to generate Swagger documentation using the `next-swagger-doc` plugin, it expects a valid OpenAPI definition as input. If the input is invalid, undefined, or null, the plugin throws this error. But what exactly is causing this input to be invalid?

Common Causes of the Error

Before we dive into the solutions, let’s explore some common scenarios that might lead to this error:

• Invalid or missing OpenAPI definition: Make sure your OpenAPI definition is valid and properly formatted. Even a single syntax error can cause the entire process to fail.
• Incorrect plugin configuration: Double-check your `next-swagger-doc` plugin configuration to ensure it’s pointing to the correct OpenAPI definition file.
• Next.js version incompatibility: Verify that your Next.js version is compatible with the `next-swagger-doc` plugin version you’re using.
• Middleware issues: Middleware functions can interfere with the Swagger documentation generation process, leading to this error.

Solving the Error: Step-by-Step Guide

Now that we’ve identified the common causes, let’s walk through a step-by-step guide to resolve the “Next-Swagger-Doc Error: Cannot convert undefined or null to object” issue:

Step 1: Verify Your OpenAPI Definition

Open your OpenAPI definition file (usually `openapi.yaml` or `openapi.json`) and ensure it’s valid and properly formatted. You can use online tools like Swagger Editor or API documentation tools like Stoplight to validate your OpenAPI definition.

openapi: 3.0.0
info:
  title: My API
  description: My API description
  version: 1.0.0
paths:
  /users:
    get:
      summary: Get users
      responses:
        200:
          description: Users list
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/User'
components:
  schemas:
    User:
      type: object
      properties:
        id:
          type: integer
        name:
          type: string

Step 2: Configure the next-swagger-doc Plugin

Make sure your `next-swagger-doc` plugin configuration is correct. Update your `next.config.js` file to point to the correct OpenAPI definition file:

module.exports = {
  //...
  plugins: [
    {
      name: 'next-swagger-doc',
      options: {
        apiDefinition: './openapi.yaml', // Update the path to your OpenAPI definition file
      },
    },
  ],
};

Step 3: Check Next.js Version Compatibility

Verify that your Next.js version is compatible with the `next-swagger-doc` plugin version you’re using. You can check the compatibility matrix on the `next-swagger-doc` GitHub page.

Step 4: Inspect Middleware Functions

If you’re using middleware functions in your Next.js project, review them to ensure they’re not interfering with the Swagger documentation generation process. Look for any middleware functions that might be modifying the OpenAPI definition or the plugin configuration.

import { NextApiRequest, NextApiResponse } from 'next';

const MyMiddleware = (req: NextApiRequest, res: NextApiResponse) => {
  // Ensure your middleware doesn't modify the OpenAPI definition or plugin configuration
  return NextDelegate(req, res);
};

Step 5: Restart Your Development Server

After making the necessary changes, restart your development server to re-run the Swagger documentation generation process:

npm run dev

Step 6: Verify Your Swagger Documentation

Navigate to `http://localhost:3000/api/docs` (or the path you’ve configured for your Swagger documentation) to verify that the Swagger UI is generated successfully.

Step	Action	Verify
1	Verify OpenAPI definition	OpenAPI file syntax error-free
2	Configure next-swagger-doc plugin	Plugin configuration updated
3	Check Next.js version compatibility	Compatible versions
4	Inspect middleware functions	Middleware functions don’t interfere with Swagger generation
5	Restart development server	Swagger documentation generated successfully
6	Verify Swagger documentation	Swagger UI available at configured path

Conclusion

Congratulations! You’ve successfully resolved the “Next-Swagger-Doc Error: Cannot convert undefined or null to object” issue. By following this step-by-step guide, you’ve ensured that your OpenAPI definition is valid, your plugin configuration is correct, and your middleware functions don’t interfere with the Swagger documentation generation process.

Remember, a well-documented API is essential for a great developer experience. With `next-swagger-doc` and a little patience, you can create stunning Swagger documentation for your Next.js project.

Final Tips

• Keep your OpenAPI definition up-to-date and synchronized with your API changes.
• Regularly test your Swagger documentation to ensure it’s generated correctly.
• Explore other `next-swagger-doc` features, such as customizing the Swagger UI or generating client code.

Now, go forth and conquer the world of API documentation!

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