Node.js module to sniff the bytes from a stream for mimetype detection

sniff.js

'use strict'

import stream from 'stream'

export default function sniff(rs, sniffLength = 16384) {
  return new Promise((resolve, reject) => {

    let buffer = Buffer.from([])
    let ws = new stream.Writable()

    ws._write = (chunk, encoding, cb) => {
      if (chunk) {
        buffer = Buffer.concat([buffer, chunk])

        if (Buffer.byteLength(buffer) > sniffLength) {
          rs.unpipe(ws)
          return resolve(buffer)

        }
      }

      cb()
    }

    ws.on('finish', () => {
      resolve(buffer)
    })
    ws.on('error', reject)

    rs.pipe(ws)
  })
}

Takes in a readable stream and then returns a promise that resolves to the first 16384 bytes. Can be used for sniffing the mimetype.