If you are looking for a solution in JS npm cli then,
To concats a list of videos together is easy using ffmpeg package with transitions.
package on npm! npm install ffmpeg-concat
Usage: ffmpeg-concat [options] <videos...>
Options:
-V, --version output the version number
-o, --output <output> path to mp4 file to write (default: out.mp4)
-t, --transition-name <name> name of gl-transition to use (default: fade)
-d, --transition-duration <duration> duration of transition to use in ms (default: 500)
-T, --transitions <file> json file to load transitions from
-f, --frame-format <format> format to use for temp frame images (default: raw)
-c, --concurrency <number> number of videos to process in parallel (default: 4)
-C, --no-cleanup-frames disables cleaning up temp frame images
-O, --temp-dir <dir> temporary working directory to store frame data
-v, --verbose enable verbose logging from FFmpeg
-h, --help output usage information
Example:
> ffmpeg-concat -t circleopen -d 750 -o huzzah.mp4 0.mp4 1.mp4 2.mp4
Documentation: https://www.npmjs.com/package/ffmpeg-concat
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