You're sending JSON:

return res.status(200).json(imgProcessed)

That means you're sending text, not binary data.

You probably wanted to send just the data:

return res.status(200).send(imgProcessed)
// --------------------^^^^

I'm assuming imgProcessed is a Buffer; if not you want to make it one. See the Express.js documentation for send. (It will handle sending the Content-Type: application/octet-stream header for you when you give it a Buffer.)

Please visit this answer https://stackoverflow.com/a/12776856/16315663 to retrieve GZIP data from the response.

Assuming, You have already retrieved full data as UInt8Array.

You just need the UInt8Array as String

const jsonString = Buffer.from(dataAsU8Array).toString('utf8')

const parsedData = JSON.parse(jsonString)

console.log(parsedData)

Edit

Here is what worked for me

const {request} = require("https")
const zlib = require("zlib")


const parseGzip = (gzipBuffer) => new Promise((resolve, reject) =>{
    zlib.gunzip(gzipBuffer, (err, buffer) => {
        if (err) {
            reject(err)
            return
        }
        resolve(buffer)
    })
})

const fetchJson = (url) => new Promise((resolve, reject) => {
    const r = request(url)
    r.on("response", (response) => {
        if (response.statusCode !== 200) {
            reject(new Error(`${response.statusCode} ${response.statusMessage}`))
            return
        }

        const responseBufferChunks = []

        response.on("data", (data) => {
            console.log(data.length);
            responseBufferChunks.push(data)
        })
        response.on("end", async () => {
            const responseBuffer = Buffer.concat(responseBufferChunks)
            const unzippedBuffer = await parseGzip(responseBuffer)
            resolve(JSON.parse(unzippedBuffer.toString()))
        })
    })
    r.end()
})

fetchJson("https://wiki.mozilla.org/images/f/ff/Example.json.gz")
    .then((result) => {
        console.log(result)
    })
    .catch((e) => {
        console.log(e)
    })

Use Buffer.from, as in:

...
var imgData = ctx.getImageData(0,0,250,300);
var data = imgData.data;
var buf = Buffer.from(data);

I've been able to write to stdout using the fs phantomjs module, thus being able to write as binary:

var fs = require('fs');
var stream = fs.open('/dev/stdout', 'wb');
stream.write(String.fromCharCode(128));
stream.flush();

It is still pretty slow for large volume of data, for some reason.