Recently, while working on a Cross-Origin Resource Sharing (CORS) issue, I encountered a perplexing problem. Despite using the cors package with Express.js, my API was not sending the Access-Control-Allow-Origin header. This is a common issue developers face, so I decided to share my experience and solution.

The Initial Configuration

Here’s what my initial CORS configuration looked like in my Express.js application:

app.use(cors({
    origin: "https://domain-a.com/"
}));

The Problem

Given the above configuration, I expected that requests from https://domain-a.com would be allowed. By default, the cors package allows all HTTP methods. However, while the API was correctly sending the Access-Control-Allow-Methods headers, the Access-Control-Allow-Origin header was conspicuously absent. This resulted in the infamous CORS error message in the browser.

Diagnosis and Troubleshooting

Initially, I was confident that my configuration was correct. The domain matched, and there were no apparent issues with the setup. However, the error persisted. After several attempts to debug the issue, including:

• Verifying the request headers
• Ensuring the server was running correctly
• Double-checking the cors package documentation

I still couldn’t pinpoint the problem.

The Discovery

After much trial and error, I finally discovered the issue. As trivial as it may seem, the problem was with the trailing slash in the origin configuration. The origin should have been https://domain-a.com instead of https://domain-a.com/.

app.use(cors({
    origin: "https://domain-a.com"
}));

The Solution

By simply removing the trailing slash, everything started working as expected. The API correctly sent the Access-Control-Allow-Origin header, and the CORS error disappeared.

Conclusion

This experience taught me a valuable lesson: sometimes the simplest mistakes can cause the most frustrating issues. When dealing with CORS configuration in Express.js (or any framework), ensure that the origin parameter is precisely specified without unnecessary characters like trailing slashes.

Here’s the corrected CORS configuration for reference:

app.use(cors({
    origin: "https://domain-a.com"
}));

This minor adjustment made all the difference, resolving the issue and allowing my application to function correctly across different domains.

Key Takeaways

• Always double-check your CORS configuration, especially the origin parameter.
• Even small mistakes, such as a trailing slash, can lead to significant issues.
• Don’t be afraid to revisit the basics when troubleshooting complex problems.

I hope this helps others who might face similar CORS issues. Happy coding!