Create an etag for an S3 object. Assumes object is uploaded with exact chunk size.

s3-etag.js

const {createReadStream} = require('fs')
const {createHash} = require('crypto')

module.exports = (path, chunkSize = 5242880) => {
  return new Promise((resolve, reject) => {
    const hashes = []
    const options = {highWaterMark: chunkSize}
    const rs = createReadStream(path, options)

    rs.on('error', reject)

    rs.on('data', data => {
      const hash = createHash('md5').update(data).digest()
      hashes.push(hash)
    })

    rs.on('end', () => {
      if (hashes.length < 2) return resolve(hashes[0].toString('hex'))

      const data = Buffer.concat(hashes)
      const hash = createHash('md5').update(data).digest('hex')

      resolve(`${hash}-${hashes.length}`)
    })
  })
}