how can i use worker?

How can i use inner function such as createWorker? my version is 0.8.5

I want to concat some videos with same format, I use concat command with the reference of
[https://github.com/ffmpegwasm/ffmpeg.wasm/blob/master/examples/browser/concatDemuxer.html](url)

but it can only play the first paragraph and then stop which is normal QuickTime player.(every video can play normally alone),I use copy command to ensure the result is right.（copy can drop the wrong frame?）

So I think if it can work normally when i use concatDemuxer?

Looking forward to your reply～