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!
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