Streaming uploads through Koa.

app.js

import Koa from 'koa'
import parse from './busboy'
import AWS from 'aws-sdk'

const app = new Koa()

const s3 = new AWS.S3({ 
  params: { Bucket: 'myBucket' } 
})

// Assuming this is a route handler.
app.use(async (ctx) => {
  const { file, filename } = await parse(ctx.req)
  // `file` is a Stream. Pass this to S3, Azure Blob Storage or whatever you want.
  // `filename` is the file name specified by the client.
  const result = await s3.upload({
    Key: filename,
    Body: file
  }).promise()
  
  ctx.body = result
})

busboy.js

import Busboy from 'busboy'

/**
 * Parses a single file from a Node request.
 *
 * @param  {http.IncommingRequest} req
 * @return {Promise<{ file: Stream, filename: string>}
 */
export default function parse (req) {
  return new Promise((resolve, reject) => {
    const busboy = new Busboy({
      headers: req.headers,
      limits: {
        files: 1 // allow only a single upload at a time.
      }
    })

    busboy.once('file', _onFile)
    busboy.once('error', _onError)
    req.pipe(busboy)

    function _cleanup () {
      busboy.removeListener('file', _onFile)
      busboy.removeListener('error', _onError)
    }

    function _onFile (fieldname, file, filename) {
      _cleanup()
      resolve({ file, filename })
    }

    function _onError (err) {
      _cleanup()
      reject(err)
    }
  })
}

client.js

// `file` is a DOM File object.
function upload (file) {
  const data = new window.FormData()
  data.append('file', file, file.name)
  return axios.post('/upload', data)
}

Thanks, this is really handy for me.
But I have one question: Why do you remove listeners after handling event? Firstly you subscribe on once, secondly your function must be garbaged after run. Or am I wrong?

@akellbl4 because the events are mutually exclusive, so if one triggers the other won't, and you might risk a dangling handler. This might be unnecessary, but I do it just in case. Better safe than sorry.