Скачать или смотреть How to Fix the ERR_HTTP_HEADERS_SENT Error in Node.js When Refreshing Pages

A detailed guide on resolving the `Cannot set headers after they are sent to the client` error in Express.js applications. Learn how to debug your Node.js application effectively.
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This video is based on the question https://stackoverflow.com/q/69971980/ asked by the user 'DrakeColeman' ( https://stackoverflow.com/u/15289840/ ) and on the answer https://stackoverflow.com/a/69973476/ provided by the user 'Elzohary' ( https://stackoverflow.com/u/15736486/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the ERR_HTTP_HEADERS_SENT Error in Node.js

If you're a Node.js developer, you might have encountered the baffling error message: Cannot set headers after they are sent to the client. This usually happens while using frameworks like Express.js and may cause your server to behave unpredictably, or even crash. In this post, we will explore the causes of this error and how you can fix it.

What Causes the Error?

The ERR_HTTP_HEADERS_SENT error typically arises when an attempt is made to send multiple responses for the same request. In a web application, especially when using asynchronous programming patterns, it’s crucial to ensure that every request receives only one response. Here are common triggers for this error:

Calling the response method (like .send(), .json(), or .redirect()) multiple times within a single request handler.

Falling into unexpected asynchronous behavior where a response is attempted after the initial one has already been sent.

Real-world Scenario: The Refresh Bug

In the scenario presented, the user initially logs in successfully; however, a page refresh leads to crashes or errors. This occurs because the application attempts to process the user session and the database queries incorrectly, resulting in multiple responses being sent.

Analyzing the Code

Let's take a closer look at the code snippet shared by the user. The following block from the passport.deserializeUser() function is where the issue lies:

[[See Video to Reveal this Text or Code Snippet]]

In this code, the done function is called twice:

Once within the nested callback from findById().

Another time within the .then() block.

By sending two responses, the application is violating its requirement to send only a single response per request, thus leading to the ERR_HTTP_HEADERS_SENT error.

The Solution

To resolve this issue, you need to ensure that the done callback is called only once. Here’s the corrected version of the deserializeUser function:

[[See Video to Reveal this Text or Code Snippet]]

Step-by-Step Breakdown:

Remove the .then() Block: Since you’re already calling done(err, user) inside the callback of findById(), there’s no need for the .then() block.

Ensure One Response: By returning control to done() only after the database query completes, you prevent the server from attempting to respond multiple times.

Conclusion

The ERR_HTTP_HEADERS_SENT error can be frustrating, especially with environments that rely on asynchronous operations like Node.js. Ensuring that a single response is sent for any request will help you maintain a stable and functional application. With the adjustments explained above, you can efficiently manage user authentication and conditionally respond based on the user's session state.

By following these guidelines, you should be able to resolve this pesky error and reinforce the reliability of your Node.js application. Remember, debugging always starts with carefully tracing where multiple responses might be occurring. Happy coding!