Node.js http.serverResponse.headerSent Property

13 Feb 2025 | 4 min read

In this article, we will discuss the Node.js httpServerResponse.headersSent property with its syntax, parameter, and examples.

What is the Node.js httpServerResponse.headersSent property?

We can find whether or not the header has been sent using the httpServerResponse.headersSent application programming interface of class ServerResponse, which is provided by the HTTP module.

The headerSent boolean value in the Node.js http.ServerResponse class indicates whether or not the response headers have already been sent to the client. When the headers are sent, usually just before writing the first response body chunk or just after calling the res.writeHead() function, this property becomes true.

The http module's ServerResponse object is used to transmit responses to the client. It is an illustration of an instance of HTTP. For interacting with the sent response, the ServerResponse class provides properties and methods.Among these properties is headersSent.

Syntax:

It has the following syntax:

Parameters:

There are no parameters that can be passed in for this property.

Return Value:

If and only if headers were sent; if not, this property returns false.

Example 1:

Let us take an example to illustrate the Node.js httpServerResponse.headersSent property.

Output:

 
Server running at http://localhost:3000/

Explanation:

In this example, headers are only set in situations where they haven't been sent yet because of the if (!res.headersSent) condition.
If the headers have not been sent yet, they are implicitly sent at the time the res.write() method sends a portion of the response body.
The res.writeHead() method sends response headers explicitly with a status code.

Example 2:

Let us take another example to illustrate the Node.js httpServerResponse.headersSent property.

Output:

 
Server is running at port 4000...
Result has been displayed…
Server has been closed

Example 3:

Let us take another example to illustrate the Node.js httpServerResponse.headersSent property.

Output:

 
Server is running at port 5000...

Explanation:

In this example, the port number of 5000 was defined.
Each incoming request's URL and request method were noted.
Before setting headers or composing the response, conditional checks were used to see if headers had been sent.
After that, used the proper status codes and messages for various HTTP methods (such as GET, POST, and others).
The headers are only set if they haven't been sent yet.
The request method indicated that a JSON response was sent.
Closed the server after sending the response.

Conclusion:

In conclusion, the headersSent property in the Node.js http.ServerResponse class is an important aspect of HTTP response handling. It allows developers to ensure that response headers are properly set before any response body content is written. This feature is critical because it can cause runtime errors by throwing an exception if headers are changed after they have been sent. Developers can leverage the headersSent attribute for a more robust and error-free approach towards managing responses in their Node.js applications, thus upholding their reliability and integrity. This particular capability underscores Node.js's attention to detail and emphasis on control, ensuring that developers have at their disposal all tools necessary for building high-performance network applications.

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