file-upload-server | Upload file to server by HTTP Post server | HTTP library

You need to set an upload limit for your express.json() middleware since you are using 'Content-Type': 'application/json' header in the post request.

Try replacing router.use(express.json()) with router.use(express.json({ limit: '50mb'})

Documentation

The error Unexpected token < in JSON at position 0' happens if you try to parse a string with response.json() which is not formatted as json. You can reproduce it in a one liner in nodejs as JSON.parse('I am no json...')

If you call response.json() on the client , it has to be properly formatted json or it result in an error. For doing so, use res.json() instead of res.send().

You should also make sure to always respond to the client, so change the line if(err) throw err; to