Express framework, error: "Cannot set headers after they are sent to the client”, solution summary Original

Problem description

"Cannot set headers after they are sent to the client" This is an error caused by the code trying to set the HTTP header after sending the response. This means that the function has sent one or more responses (such as an HTTP response header, an HTTP body, or a JSON string) and then attempts to send another response.

Problem analysis

This error may be caused by the code having an asynchronous callback and sending the response repeatedly or sending the response to the client through two or more code paths. As shown below:

('/example', function(req, res) {
    ('Hello World!');
    (200).json({ message: 'Invalid request' }); // Error: Cannot set header});

Solution

To resolve this issue, you can make sure that a piece of code passes only once and avoid duplicate responses. It is best to use when responding in asynchronous operationsPromiseorasync/awaitto handle the control process.

In addition, the error can be reflected by returning the status code or a custom error message without passing()or()etc. to send responses to avoid duplicate responses and header modifications.

Here are examples of improvements to the above problem:

('/example', async (req, res) =&gt; {
  try {
    const result = await someAsyncOperation();
    // Process the result and send it as a response to the client    (200).json(result);
  } catch (error) {
    // When an error occurs, a custom error message is returned    (500).json({ message: 'Error occurred while processing request' });
  }
});

In this example, we useasync/awaitKeywords to handle asynchronous operations. If the operation completes successfully, some data will be returned, otherwise, the exception will be caught and a custom error message will be sent.

Finally, we send only once when sending a response to the client.

Supplement (2 common problem solutions online):

1. Req-res has not formed a closed loop, just remove next

('/',function(req,res,next){
    ("Get a get req");
    ("hello lcq!");
    next();
}

2. Asynchronous and synchronization issues:

Problem description

In the NodeJs+express+mongoDB interface, an error will be reported when writing `` in a loop (Cannot set headers after they are sent to the client)

Solution

Define a variable outside the loop as `false`
Where to write `` in the loop makes it true`
Write `` when judging that the variable is `true` outside the loop

Specific code implementation

var isShow = false;   //Define the switch variable//loadcurr is an array, the specific content is omitted(item =&gt; {
    ({             //Stock is used to connect to database collection       _id: ObjectId(item._id)
        }, {
            $set: {
            //The content to be modified is omitted here                。。。。。。
            }
        },
        function (err, data) {
            if (err) {
                (err)
            } else {
            //It should be written here, but in order to solve the error, the variable defined above is true                isShow = true
                ('1', isShow);
            }
        })
})
('2', isShow);
if (isShow == true) {
    ({
        status: "200"
    })
}

This will successfully solve the problem of Cannot set headers after they are sent to the client! ! !

Notice:

If the above code is executed in order, it is printed first('2', isShow);Print afterward('1', isShow);, indicating that there is asynchronous, so it can be solved by asynchronously. The specific implementation is as follows (if the order is executed first('1', isShow);After execution('2', isShow);That's right):

If the return value is not promise, use the following method

var isShow = false;
    (async function(){
        await new Promise((resolve, reject) =&gt; {
            //loadcurr is an array, the specific content is omitted            (item =&gt; {
                //Stock is used to connect to database collection                ({
                    _id: ObjectId(item._id)
                }, {
                    $set: {
                        //The content to be modified is omitted here                        。。。。。。
                    }
                },
                function (err, data) {
                    if (err) {
                        (err)
                    } else {
                        resolve(isShow = true)
                        ('1', isShow);
                    }
                })
            })
        })
        ('2', isShow);
            if (isShow == true) {
                ({
                    status: "200"
                })
            }
    })()

If the return value is promise, use the following method

var isShow = false;
    (async function(){
        //loadcurr is an array, the specific content is omitted        await (item =&gt; {
            //Stock is used to connect to database collection            _id: ObjectId(item._id)
        }, {
              $set: {
              //The content to be modified is omitted here              。。。。。。
           }
         },
            function (err, data) {
                if (err) {
                    (err)
                } else {
                    isShow = true
                    ('1', isShow);
                }
            })
        })
        ('2', isShow);
            if (isShow == true) {
                ({
                    status: "200"
                })
            }
    })()

The above is to solve the loop and write itReport an error problem and solve asynchronous problem!