Resolving a 404 Error When Deploying a Node.js Application on Vercel

The Issue

When deploying a Node.js server application on Vercel, I encountered a frustrating 404 error. Despite the application running flawlessly on my local machine, after deployment on Vercel resulted in the dreaded "404: NOT_FOUND" error, leaving me puzzled and unsure about the root cause.

Challenges

I was in the process of testing my Node.js application by deploying it on Vercel. The project was well-configured and worked perfectly on my local environment. However, after deploying it on Vercel, I encountered a persistent 404 error.

This error indicated that Vercel was unable to find the appropriate endpoint for my application, despite all my configurations appearing correct. The error message displayed on Vercel was:

At first, I was unsure why this was happening. I reviewed my project structure, rechecked my configurations, and even redeployed the application multiple times, but the issue persisted. The fact that everything worked fine locally added to my confusion.

The Struggle

Determined to resolve this issue, I turned to various resources. I scoured the official Vercel documentation, read through community posts, explored articles, and even sought advice from AI platforms like ChatGPT and Phind.com. Despite all this effort, the solution wasn’t immediately apparent.

Through extensive troubleshooting, I finally discovered the root cause: the structure of my project didn’t align with Vercel’s requirements for Node.js applications. Specifically, Vercel expected a certain folder structure and entry file setup that I hadn’t implemented correctly.

Steps I Followed

The issue boiled down to the following:

1. Project Structure: Vercel requires the Node.js application to have an /api directory containing an index.js file, which acts as the entry point for the application. My project had a server.js file at the root level instead of an index.js file within the /api directory.

2. Configuration: For the deployment to work correctly, I needed to create a vercel.json file at the root level of my project. This configuration file is crucial as it instructs Vercel on how to handle routing and where to find the entry point for the application.
   The vercel.json file I created contained the following configuration:

{ "version": 2, "rewrites": [{ "source": "/(.*)", "destination": "/api" }] }

This configuration ensures that all requests are routed through the /api directory, where the index.js file resides.

Resolution and Solution

By restructuring my project to place the index.js file within the /api directory and adding the vercel.json file with the correct routing configuration, I was able to resolve the 404 error. After making these changes, I redeployed the application on Vercel, and it worked flawlessly. The endpoint was successfully accessible, and the application was now live without any errors.

Understanding Vercel's Architecture

To understand why these changes worked, we need to delve into how Vercel handles Node.js deployments under the hood.

1. Serverless Functions and Routing: Vercel is built around the concept of serverless functions. When you deploy a Node.js application on Vercel, each API route is treated as a serverless function. These functions are deployed in the /api directory, and Vercel automatically identifies the routes based on the structure of this directory.
   When a request is made, Vercel’s routing layer directs it to the appropriate serverless function. If the folder structure is not as expected, Vercel will not find the required entry point, resulting in a 404 error.

2. Automatic Routing and the vercel.json File: Vercel uses automatic routing to map URLs to specific files in the /api directory. If the project structure deviates from this convention, Vercel cannot correctly route requests to the application’s endpoints. The vercel.json file allows you to customize this routing behavior.
   By specifying rewrites in the vercel.json file, you inform Vercel to reroute all incoming requests to the /api directory, ensuring that your serverless functions are triggered correctly.

3. Deployment Process: During deployment, Vercel packages each function in the /api directory as an individual serverless function. If the structure does not align with Vercel's expectations, the deployment will succeed, but the application will fail to route requests correctly, resulting in a 404 error.

4. Caching and Content Delivery: Vercel also utilizes a globally distributed CDN to cache and serve your application’s content. The rewrites specified in the vercel.json file ensure that the correct serverless function is invoked and cached responses are invalidated as needed.

How These Changes Worked

These changes addressed the core issue at multiple levels:

1. Proper Routing: By placing the index.js file in the /api directory, I ensured that Vercel could correctly identify the entry point for my Node.js application. This aligned the project structure with Vercel’s expectations, allowing it to map incoming requests to the correct serverless function.

2. Custom Routing with vercel.json: The vercel.json file provided explicit instructions to Vercel’s routing layer, ensuring that all requests are rerouted to the /api directory. This customization is essential when your project structure differs from the default expectations.

3. Optimized Deployment: Vercel’s deployment process was now able to package and deploy the serverless functions correctly. By adhering to the expected architecture, I avoided misconfigurations that could lead to deployment issues or runtime errors.

4. Performance and Caching: With the correct routing in place, Vercel’s CDN could cache responses and deliver content efficiently, improving the overall performance of the deployed application.

Conclusion

Understanding Vercel’s architecture and how it handles Node.js deployments is crucial for resolving deployment issues like the 404 error. By aligning your project structure with Vercel’s expectations and using a vercel.json file to customize routing, you can ensure a smooth deployment process.

If you face a similar problem, consider the underlying architecture and routing requirements. This approach should help you resolve the issue and get your Node.js application running smoothly on Vercel.