const frames = ['frame1.jpg', 'frame2.jpg', ...]

const conv = ffmpeg() // create converter
const input = conv.input({f: 'image2pipe', r: 30}) // create input writable stream
conv.output('out.mp4', {vcodec: 'libx264', pix_fmt: 'yuv420p'}) // output to file

// for every frame create a function that returns a promise
frames.map(filename => () =>
  new Promise((fulfill, reject) =>
    s3
      .getObject({Bucket: '...', Key: filename})
      .createReadStream()
      .on('end', fulfill) // fulfill promise on frame end
      .on('error', reject) // reject promise on error
      .pipe(input, {end: false}) // pipe to converter, but don't end the input yet
  )
)
// reduce into a single promise, run sequentially
.reduce((prev, next) => prev.then(next), Promise.resolve())
// end converter input
.then(() => input.end())

conv.run()