express route to serve mp4 video files from AWS s3

s3-mp4-video.ts

// npm i express@^4.15.2 @aws-sdk/client-s3@^3.540.0

import { Request, Response, Router } from 'express'
import { GetObjectCommand, HeadObjectCommand, S3Client } from '@aws-sdk/client-s3'
const router = Router()

/** stream a video from S3 */
router.get('/:filepath(*.mp4)', async (req: Request, res: Response) => {
  const filepath = req.params.filepath || req.params[0]
  const range = req.headers.range;
  if (!range) {
    res.status(416).send({err: 'Wrong range'})
    return
  }

  const credentials = {
    accessKeyId: process.env.S3_ACCESS_KEY_ID || '',
    secretAccessKey: process.env.S3_SECRET_ACCESS_KEY || '',
  }
  const region = process.env.S3_REGION
  const bucket = process.env.S3_BUCKET
  if (!credentials.accessKeyId || credentials.secretAccessKey || !region || !bucket) {
    console.error('Unexpected error. At least one of S3_ACCESS_KEY_ID, S3_SECRET_ACCESS_KEY, S3_REGION, or S3_BUCKET is unset.')
    res.status(500).send({err: 'Unexpected error. See server log for details.'})
    return
  }

  const client = new S3Client({region, credentials})

  const headRes = await client.send(new HeadObjectCommand({ Bucket: bucket, Key: filepath }))
  const contentLength = headRes.ContentLength
  const lastModified = headRes.LastModified?.toUTCString()
  const etag = headRes.ETag
  if (!contentLength) {
    res.status(404).send({err: 'Not found'})
    return
  }

  const [starts, ends] = range.replace(/bytes=/, '').split('-')
  const start = parseInt(starts, 10)
  const end = ends ? parseInt(ends, 10) : contentLength - 1

  const getRes = await client.send(new GetObjectCommand({ Bucket: bucket, Key: filepath, Range: range }))
  res.status(206)

  Object.entries({
    'Accept-Ranges': 'bytes',
    'Cache-Control': 'no-cache',
    'Connection': 'keep-alive',
    'Content-Length': (end-start)+1,
    'Content-Range': `bytes ${start}-${end}/${contentLength}`,
    'Content-Type': 'video/mp4',
    'ETag': etag,
    'Keep-Alive': 'timeout=5',
    'Last-Modified': lastModified,
  }).forEach(([k,v])=>v && res.setHeader(k,v))
    
  if (!getRes.Body) {
    res.status(502).send({err: 'Failed to get content from S3.'})
    return
  }
  // casting since pipe is unexpectedly undefined for GetObjectCommandOutput.Body
  (getRes.Body as ({pipe:(res:Response)=>void})).pipe(res)
})