Where do you even start with err_http_headers_sent when you are using Node.js with Express and you have encountered the dreaded error [err_http_headers_sent]: cannot set headers after they have been sent to the client. The common causes of this issue, such as invoking res.writeHead, res.write, or res.redirect, will be discussed in this article.

When servers send headers after the client has received the message, a problem arises. Node JS applications provide many responses to a single request, for example, by calling the request twice rather than using a single request and a single response using the res.send headers method.

Know Why You Keep Getting “Can’t Set Headers After They Have Been Sent To The Client Nodejs” Error?

The three most frequent causes of this problem when Express is used as a web server and Node.js is used as the request handler function nodejs response.end are listed below:

  • The res.write command you left too soon
  • Adding extra data after calling res.redirect
  • Calling after the function body or function caller has returned a response
  • Look for redundant function calls

Continue reading for examples of how each of these three typical causes could arise, as well as instances of improper code and how to repair it, in order to stop receiving error [err_http_headers_sent]: Once being sent to the client, headers cannot be changed.

The homepage of your website’s ‘err_http_headers_sent’ function will trigger a response. Now let’s go over the typical situations that result in Node.js and Express throwing the error: can’t set headers after they are sent. After you have resolved this issue, be sure to read more of my Node.js tutorials, which will help you further develop your knowledge of JavaScript and Node.js and prevent you from ever seeing the error “can’t set headers after they are sent”!

Calling Res.Writehead Of The Request Handler Code Too Soon

If you call res.setHeader many times in your Express route method, the middleware item will output the error code ‘err_http_headers_sent’. Express has already created headers for the response to your call as soon as you call res.writeHead or res.write. The error message for any subsequent calls to res.setHeader is “cannot set headers after they have been sent to the client nodejs.” Here is an illustration of what to search for when setting headers that can have unintentionally been called twice from a single request that can output two retorts to the client:

  1. get(‘/’,function (req,res,next){// Call to some middleware
  2. // middleware code calls: res.writeHead();
  3. setHeader(‘Content-Type’, ‘text/html’);
  4. // Exception thrown after setHeader call
  5. });

Calling Res.Redirect And Then Adding More Data

Express has completed the request when you call res.redirect. Any additional data setting done before Express does the redirect will likewise prevent headers from being changed after they have been sent. Let’s see an illustration of how this could occur from the response header as the client error since there were many responses in your callback functions:

  1. post(‘/’,function (req,res,next){// Do something with the post data
  2. redirect(‘/thankyou’);
  3. // Redirect to thank you page
  4. // Additional function call that causes an error
  5. next()
  6. });

Calling Again After Receiving A Response

Calling next after res.end has been called is the last frequent scenario I am aware of. This happens frequently when middleware that you are unaware of calls res.end.

  1. get(‘/’,function (req,res,next){// Some middleware that executes
  2. // res.end
  3. // Additional function call that causes an error
  4. next();
  5. });

Make sure the call to next comes before the call to res.end to resolve this. In a perfect world, the middleware either doesn’t call res.end and leaves it up to your code to finish the request OR it calls next for you before using res.end. You should be able to resolve the dreaded Can’t render headers once they are transmitted using one of these three suggestions.

Before An Error Occurs, Look For Duplicate Function Calls.

Can’t add a header while transmitting, error. Errors typically happen after multiple requests have been answered. Using this function will allow you to avoid the following routines if the request was redirected after you checked: When an error from the actual function happens, you should check the set headers using the main response method, which is described below.

Node Error: An Understanding [ERR_HTTP_HEADERS_SENT]

The following are possible effects of a runtime fault for Node JS developers: Can’t set “Headers” after sending them to a user with frequent problems in HTTP HEADER_SEND. Finding the cause of the code’s issue and the manner in which the response object contributed to it becomes increasingly challenging.

This issue can be very aggravating, and you might experience a slight variant of it. As a result, any of the following possibilities could occur:

  • error [err_http_headers_sent]: headers cannot be changed after being sent to the client.
  • error [err_http_headers_sent]: unable to set headers after the client has received them
  • unable to change headers once they have been sent to the client HTTP-server
  • error [err_http_headers_sent]: Upon sending headers to the client, they cannot be changed

Look For Questionable Middleware

Objects should be appended to the Server in the app-use method.Prototype.The connect interface’s stack. The server moves the stack and invokes the request-response method as soon as it receives the requested request. An error is raised if an intermediate object calls respond() and then sends a message in the response body or headers without first calling respond().

What causes the issue “error: cannot set headers after they are sent to client” to appear in express?

A wrapper for the HTTP module is expressed. It makes it easier to create custom middleware and routes. Middleware is a feature that regulates communication between request and response objects.

Express determines the type of middleware based on the number of given parameters. An example of a middleware that takes callbacks with request and response parameters is route middleware. The request, response, and next arguments are frequently sent to bespoke middleware. Last but not least, an error handler typically receives the parameters error, request, response, and next.

A request is made by the client. Through the request argument, express gets the request and processes it. After processing the request, it uses the response object handler to send feedback. One HTTP cycle has now been finished.

One of the following methods: json(), send(), sendStatus(), redirect(), or render() is sent by express to conclude an HTTP cycle.

JSON is sent to the client through json(). Send() typically sends objects, strings, or buffers. The response status code is set by sendStatus(), which also delivers its string representation. For example, an OK status code is 200.

How to check whether headers are transmitted

To find out if the headers have been sent or not, check res.headersSent. It has already transmitted the headers if it is set to true. Checking this response attribute will avoid multiple headers. You can use return statements in each and every response send function as shown below to resolve this issue.

app.get(‘/’, function(req, res){

res.send(‘Hello World!’);

if(res.headersSent !== true) {

res.send(‘Hello World!’);

}

});

Unable To Change Headers Once They Have Been Sent To The Client – When It Occurs

When an express.js application attempts to send numerous answers in response to a single request, it encounters the “[ERR_HTTP_HEADERS_SENT]: unable to change headers once they have been sent to the client” error. Take the following precautions to fix the issue:

  • Avoid sending two replies to one request.
  • After sending a reply, refrain from using res.setHeader().
  • Never call next after the function body has delivered a response.

1.     Avoid sending two replies to one request.

A single answer should always be sent in response to a single request from the node server. Redirect(), Render(), transmit(), and Res.Json() are frequently used functions that transmit responses.

When you submit responses inside of many conditional checks, which is a common circumstance, an issue occurs. There is a potential that you will send several responses if there is a conditional logic problem.

app.get(‘/’, function(req, res){

    const message = “Hello world!”;

    if(message != “”){

        res.send(message);

    }

    // Here throws an error: Can’t set headers after they are sent to the client

    res.send(“No message”);

})

The res.send() function was called twice in the code above due to a conditional logic error. A server error is thrown as a result of this.

2.     After Sending A Reply, Refrain From Using Res.Setheader().

After sending a response to the client, be sure not to use res.setHeader(). Only before res.redirect(), res.render(), res.send(), res.json(), and res.writeHead() operations can res.setHeader() be used.

For implicit headers, the res.setHeader() function sets a single header value. If this header is present in the headers to be sent, a new value will be substituted for it.

Only calls to res.writeHead(statusCode) and res.setHeader(name, value) are permitted in between. Following the writeHead() procedure, the headers are prepared for sending a response. Only res.write(data) and res.end(data) can be called thereafter. Res.writeHead() throws an error if it is called after res.setHeader().

If the current middleware function in Express does not conclude the request-response cycle, you will need to call next() to transfer control to the subsequent middleware function. If not, the request will be ignored.

3.     Never Call Next After The Function Body Has Delivered A Response.

If the current middleware function in Express does not conclude the request-response cycle, you will need to call next() to transfer control to the subsequent middleware function. If not, the request will be ignored. Make sure you’re not executing the next function after the function body has delivered a response.

An error is thrown by the middleware procedure below.

const checkSuperUser = function(req, res, next) {

    if (!req.user.isSuperUser) {

        res.redirect(‘/’); //ERROR OCCURS HERE

    }

    next();

};

Use the return statement in res.redirect(“/”) to fix the mistake in the preceding code.

const checkSuperUser = function(req, res, next) {

if (!req.user.isSuperUser) {

return res.redirect(‘/’);

}

next();

};

Conclusion

Always make sure you are providing a single response in response to a single request in order to avoid the error “[ERR_HTTP_HEADERS_SENT]: After they are sent to the client, headers cannot be changed”. Ensure that middlewares that have received a response from the client are not utilizing res. setheader() or next().