Solving the Infamous “Failed to Parse Body as FormData” Issue in Next.js 14 App Router

Are you tired of scratching your head over the frustrating “Failed to parse body as FormData” error in your Next.js 14 app router? You’re not alone! This pesky issue has been driving developers crazy, but fear not, dear reader, for we’re about to dive into the solution together.

What’s Causing This Error?

Before we dive into the fix, let’s quickly understand what’s causing this error. In Next.js 14, the app router expects the request body to be in the JSON format by default. However, when you try to send a Form Data (multipart/form-data) request, the app router gets confused and throws this error.

The Culprit: `getStaticProps` and `getServerSideProps`

The error often occurs when you’re using `getStaticProps` or `getServerSideProps` in your Next.js pages or API routes. These functions are meant to pre-render pages or run server-side logic, but they can also interfere with the request body parsing.

The Fix: Configure the App Router to Accept FormData

Now that we know the cause, let’s get to the solution! To fix this issue, you need to configure the app router to accept FormData requests. You can do this by adding a custom middleware function to your `next.config.js` file.

module.exports = {
  //...
  experimental: {
    appRouter: {
      middleware: [
        async function formDataMiddleware(req, res, next) {
          if (req.headers['content-type'] === 'multipart/form-data') {
            const formData = await parseFormData(req);
            req.body = formData;
          }
          return next();
        },
      ],
    },
  },
};

In this code snippet, we’re adding a custom middleware function that checks if the request has a `multipart/form-data` content type. If it does, we parse the FormData using the `parseFormData` function (which we’ll create next) and assign it to the `req.body` property.

The `parseFormData` Function

Create a new file (e.g., `formDataParser.js`) and add the following code:

import { IncomingForm } from 'formidable';

async function parseFormData(req) {
  const form = new IncomingForm();
  const formData = await new Promise((resolve, reject) => {
    form.parse(req, (err, fields, files) => {
      if (err) {
        reject(err);
      } else {
        resolve({ fields, files });
      }
    });
  });
  return formData;
}

export default parseFormData;

This function uses the `formidable` library to parse the FormData from the request. It returns an object with `fields` and `files` properties, which we’ll assign to the `req.body` property in our middleware function.

Example Usage

Let’s say you have an API route that accepts a file upload:

import { NextApiRequest, NextApiResponse } from 'next';

const uploadApi = async (req: NextApiRequest, res: NextApiResponse) => {
  if (!req.body) {
    return res.status(400).json({ error: 'No request body' });
  }

  const { files } = req.body;
  const file = files.file; // assuming you're sending a single file

  // process the file upload
  // ...

  return res.status(201).json({ message: 'File uploaded successfully' });
};

In this example, the API route expects the request body to contain the uploaded file. With our custom middleware function, the `req.body` property will be populated with the parsed FormData, allowing us to access the uploaded file.

Troubleshooting Tips

If you’re still encountering issues, here are some troubleshooting tips to keep in mind:

• Make sure you’ve installed the `formidable` library by running `npm install formidable` or `yarn add formidable`.
• Verify that your request is sending the correct `Content-Type` header with the value `multipart/form-data`.
• Check that your middleware function is being called by adding a console log statement or using a debugger.
• If you’re using `getStaticProps` or `getServerSideProps`, ensure that you’re not accidentally overriding the `req.body` property.

Conclusion

And that’s it! With this solution, you should be able to fix the “Failed to parse body as FormData” error in your Next.js 14 app router. Remember to configure the app router to accept FormData requests by adding a custom middleware function, and don’t forget to parse the FormData using the `parseFormData` function.

If you have any further questions or need additional assistance, feel free to ask in the comments below. Happy coding!

Keyword	Description
Getting error: Failed to parse body as FormData in Next.js 14 app router	This article provides a comprehensive solution to the “Failed to parse body as FormData” error in Next.js 14 app router, including configuring the app router to accept FormData requests and parsing the FormData using a custom middleware function.

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Having trouble with the pesky “Failed to parse body as FormData” error in your Next.js 14 app router? Don’t worry, we’ve got you covered! Here are some frequently asked questions and answers to help you debug and fix the issue.