res.send(Buffer.from(someString)) overwriting Content-Type response header

[dylan-hart]

I've got this express (4.17.1) app running on Node.js (12.14.0):

const express = require('express')
const https = require('https')
const http = require('http')

const app = express()

const JSON_STRING = `{
    "associatedApplications": [
        {
            "applicationId": "abcdef"
        }
    ]
}`

// Serve .json file.
app.use('/my-json.json', (req, res, next) => {
    res.set('Content-Type', 'application/json')
    res.send(Buffer.from(JSON_STRING))
})

// Configure SSL.
const https_options = {
  key: ...,
  cert: ...,
  ca: [
     ...
  ]
}

// Make https server.
const https_server = https.createServer(https_options, app)
https_server.listen(443)

// Make http server.
const http_server = http.createServer(app)
http_server.listen(80)

In serving the .json file, I am trying to set the Content-Type response header to application/json, but Express appears to be overwriting it with application/json; charset=utf-8.

This appears to be normal behavior usually, but the docs and this previous issue seem to say that Express shouldn't overwrite the Content-Type response header when res.send() is passed a Buffer object.

Am I missing something, or is the documentation incorrect? If the documentation is incorrect and this is normal behavior, is there another way I can prevent Express from automatically overwriting the Content-Type response header?

For extra context: this is an issue related to configuring an application's publisher domain in Azure.

[dougwilson]

I believe this is not an override like the linked issue, but is a feature of using res.set instead of res.setHeader where res.set will add the standard charset if not supplied where res.setHeader will use your header as-is.

[dylan-hart]

Thanks for your input. Using res.setHeader('Content-Type', 'application/json') seems to still produce application/json; charset=utf-8, but I will keep playing around with it and see if I can come up with something.

[dougwilson]

Ok. I will close for now until there is a reproduction we can look into to determine what is going wrong. Here is a terminal session of me trying to reproduce the issue. Perhaps you can provide something similar? Not sure if maybe there is something else at play in your setup adding the charset or something.

$ npm i express
+ [email protected]
added 50 packages from 37 contributors and audited 126 packages in 3.886s
found 0 vulnerabilities

$ node -v
v12.14.0

$ cat app.js
const express = require('express')
const http = require('http')

const app = express()

const JSON_STRING = `{
    "associatedApplications": [
        {
            "applicationId": "abcdef"
        }
    ]
}`

// Serve .json file.
app.use('/my-json.json', (req, res, next) => {
    res.setHeader('Content-Type', 'application/json')
    res.send(Buffer.from(JSON_STRING))
})

// Make http server.
const http_server = http.createServer(app)
http_server.listen(80)

$ node app &
[1] 1169

$ curl -is http://localhost/my-json.json
HTTP/1.1 200 OK
X-Powered-By: Express
Content-Type: application/json
Content-Length: 99
ETag: W/"63-IYHmhtDM3p6rsi4qSsHF0skYiXA"
Date: Sat, 21 Dec 2019 15:59:16 GMT
Connection: keep-alive

{
    "associatedApplications": [
        {
            "applicationId": "abcdef"
        }
    ]
}

You can see in the cURL output, the response header is just Content-Type: application/json