CODEWITHSUNDEEP
javascriptnode.jsexpressweb
Taher Hozefa
Taher Hozefa

Reputation: 21

node js server headers

I am currently a beginner at web-development. Therefore naturally i am using node.js as a back-end for an app that i am creating that involves getting data using REST and then searching and sorting. The part that i am stuck on is how do headers work node.js and in general on web server. The code i have below is basically setting up a server on port 3000 , and then i have a counter that counts how many times the endpoint was hit. What i dont get is the error that node throws. Therefore i tried to rewrite the headers. From the research i have done online is that once the server which is node.js in this case makes a handshake with the client, which is our front end, it sets a header. Then the client can make a request to server using api routes and get a response. What i dont understand is how headers are used in this ?

const express = require('express');
const app = express();
const port = 3000;

var counter=0;

app.get('/',(req,res) => {
    res.send("Hello World");
    console.log("First Request");
    counter++;

    if(counter%2)
    {
           res.setHeader("Content-Type", "text/html");
           res.send("Second Request");
    }


})


app.listen(port,()=>{
    console.log("App is listening on port 3000");
})

The exception it throws is this

rror [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:470:11)
    at app.get (/Users/taherhuzefa/Desktop/Personal/groupme_extension/index.js:14:16)
    at Layer.handle [as handle_request] (/Users/taherhuzefa/Desktop/Personal/groupme_extension/node_modules/express/lib/r
outer/layer.js:95:5)
    at next (/Users/taherhuzefa/Desktop/Personal/groupme_extension/node_modules/express/lib/router/route.js:137:13)
    at Route.dispatch (/Users/taherhuzefa/Desktop/Personal/groupme_extension/node_modules/express/lib/router/route.js:112
:3)
    at Layer.handle [as handle_request] (/Users/taherhuzefa/Desktop/Personal/groupme_extension/node_modules/express/lib/r
outer/layer.js:95:5)
    at /Users/taherhuzefa/Desktop/Personal/groupme_extension/node_modules/express/lib/router/index.js:281:22
    at Function.process_params (/Users/taherhuzefa/Desktop/Personal/groupme_extension/node_modules/express/lib/router/ind
ex.js:335:12)
    at next (/Users/taherhuzefa/Desktop/Personal/groupme_extension/node_modules/express/lib/router/index.js:275:10)
    at expressInit (/Users/taherhuzefa/Desktop/Personal/groupme_extension/node_modules/express/lib/middleware/init.js:40:
5)

Upvotes: 0

Views: 4651

Answers (2)

Richang Sharma
Richang Sharma

Reputation: 442

What you are doing is your are trying to send the response after it has already been sent in some cases. To clear it out,

Case 1 : When your if condition is false .

In this case the res.send('Hello World'); would have already ran, and the If condition res.send("second request") won't run. So, you won't get the error.

Case 2 : When your if condition is true.

In this case the res.send("Hello World"); will run first, that means response has already been send, now "If" condition will be true and program will send response again, which is illegal operation in express, as response is already been sent.

To solve this, change your code to this :

const express = require('express');
const app = express();
const port = 3000;

var counter=0;

app.get('/',(req,res) => {

 if(counter%2)
{
       res.setHeader("Content-Type", "text/html");
       res.send("Second Request");
}

else{
res.send("Hello World");
console.log("First Request");

}
counter++;
})


app.listen(port,()=>{
console.log("App is listening on port 3000");
})

Upvotes: 1

ellipsis
ellipsis

Reputation: 12152

Use res.write() to send multiple instances off data. res.send() ends the connections after it first sends data. With res.write() you can send data multiple times and then close the connection on your convince with res.end(). res.send() after sending data the first time closes the connection automatically. res.send() will work the first time as internally it calls res.write() followed by a res.end() and multiple times calling res.send() won't work for that reason.

var counter=0;
app.get('/',(req,res) => {
console.log("First Request");
    counter++;
  res.write("Hello World");
    if(counter%2)
    {
           res.setHeader("Content-Type", "text/html");
           res.write("Second Request");
    }


res.end();

})

Upvotes: 0

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