Troubleshooting and Fixing Common Issues in a Full-Stack Project: Next.js (Frontend) + FastAPI (Backend)

When working on a full-stack project, it’s common to encounter issues that span both the frontend and backend. In this article, we’ll walk through a real-world scenario where we faced challenges while setting up a project with Next.js (frontend) and FastAPI (backend). We’ll explore the issues, their root causes, and step-by-step solutions to resolve them.

Project Overview

The project consists of:

• Frontend: A Next.js application with Turbopack for faster development.
• Backend: A FastAPI server running with Uvicorn.
• Goal: Run both the frontend and backend simultaneously using concurrently.

Issue 1: Backend Fails to Start (ModuleNotFoundError: No module named 'backend')

Symptoms

When running the backend with uvicorn backend.main:app --reload, the following error occurs:

ModuleNotFoundError: No module named 'backend'

Root Cause

• The uvicorn command is executed from the wrong directory.
• Python cannot find the backend module because the working directory is incorrect.

Solution

1. Run the Backend from the Root Directory:
   • Ensure you’re in the root directory of your project (e.g., C:\...\chatgpt-clone).
   • Run the backend server from there:

     uvicorn backend.main:app --reload

2. Update the dev:all Script:
   • Modify the package.json script to change the directory before starting the backend:

     "dev:all": "concurrently \"cd .. && uvicorn backend.main:app --reload\" \"npm run dev\""

Issue 2: Frontend Fails to Start (error: unknown option '--turbopack')

Symptoms

When running the frontend with next dev --turbopack, the following error occurs:

error: unknown option '--turbopack'

Root Cause

• The --turbopack flag is not recognized because it’s not supported in the current version of Next.js or is incorrectly specified.

Solution

1. Use the Correct Flag:
   • The correct flag for Turbopack is --turbo, not --turbopack.
   • Update the dev script in package.json:

     "dev": "next dev --turbo"

2. Ensure Next.js Version Supports Turbopack:
   • Turbopack is supported in Next.js 13+.
   • Check your Next.js version:

     npm list next

   • If necessary, update Next.js:

     npm install next@latest

Issue 3: TypeScript Configuration Error (Configuring Next.js via 'next.config.ts' is not supported)

Symptoms

When running the frontend, the following error occurs:

Error: Configuring Next.js via 'next.config.ts' is not supported. Please replace the file with 'next.config.js' or 'next.config.mjs'.

Root Cause

• Next.js does not natively support TypeScript for configuration files (next.config.ts).

Solution

1. Rename next.config.ts to next.config.js:
   • Convert the TypeScript configuration file to JavaScript.
   • Example:

     /** @type {import('next').NextConfig} */
     const nextConfig = {
       reactStrictMode: true,
       swcMinify: true,
       experimental: {
         turbo: true,
       },
     };
     
     module.exports = nextConfig;

2. Use ts-node for TypeScript Configuration (Optional):
   • If you prefer to keep next.config.ts, install ts-node and update the dev script:

     npm install --save-dev ts-node typescript

   • Update the dev script:

     "dev": "NODE_OPTIONS=-r ts-node/register next dev --turbo"

Issue 4: Environment Variable Error on Windows ('NODE_OPTIONS' is not recognized)

Symptoms

When running the frontend on Windows, the following error occurs:

'NODE_OPTIONS' is not recognized as an internal or external command, operable program or batch file.

Root Cause

• The NODE_OPTIONS environment variable is not set correctly on Windows.

Solution

1. Use set for Windows:
   • Update the dev script for Windows:

     "dev": "set NODE_OPTIONS=-r ts-node/register&& next dev --turbo"

2. Use cross-env for Cross-Platform Compatibility:
   • Install cross-env:

     npm install --save-dev cross-env

   • Update the dev script:

     "dev": "cross-env NODE_OPTIONS=-r ts-node/register next dev --turbo"

Final Working Setup

package.json

{
  "name": "frontend",
  "version": "0.1.0",
  "private": true,
  "scripts": {
    "dev": "cross-env NODE_OPTIONS=-r ts-node/register next dev --turbo",
    "dev:all": "concurrently \"cd .. && uvicorn backend.main:app --reload\" \"npm run dev\""
  },
  "devDependencies": {
    "concurrently": "^7.0.0",
    "cross-env": "^7.0.0",
    "ts-node": "^10.9.1",
    "typescript": "^5.0.0"
  },
  "dependencies": {
    "next": "^14.0.0"
  }
}

Running the Project

1. Navigate to the root directory of your project.
2. Run the following command:

   cd frontend && npm run dev:all

Setting up a full-stack project can be challenging, especially when dealing with multiple tools and platforms. By understanding the root causes of common issues and applying the solutions outlined in this article, you can streamline your development process and avoid common pitfalls. Whether it’s fixing directory issues, resolving configuration errors, or ensuring cross-platform compatibility, these troubleshooting steps will help you build a robust and efficient development environment.

Let me know if you have further questions or run into other issues!