Download an MP4 file from a master.json's list of M4S files.

genBash.js

function genBash(masterURL) {
    var xhr = new XMLHttpRequest();
    xhr.open("GET", masterURL, true);
    xhr.addEventListener("load", function () {
        var master = JSON.parse(this.responseText);
        var baseURL = new URL(master.base_url, masterURL);

        function genBashLines(mp4, type) {
            var mp4BaseURL = new URL(mp4.base_url, baseURL);
            var segmentURLs = mp4.segments.map(segment => segment.url);
            return [`base64 -d <<< ${mp4.init_segment} > ${type}.mp4`].concat(
                mp4.segments.map(segment => `curl "${new URL(segment.url, mp4BaseURL)}" >> ${type}.mp4`)
            );
        }

        var bestVideo = master.video.sort((a, b) => b.bitrate - a.bitrate)[0];
        var bestAudio = master.audio.sort((a, b) => b.bitrate - a.bitrate)[0];

        var bashVideo = genBashLines(bestVideo, "video");
        var bashAudio = genBashLines(bestAudio, "audio");

        var bash = bashVideo.concat(bashAudio);
        bash.push("ffmpeg -i video.mp4 -i audio.mp4 -c copy vod.mp4");
        bash.push("rm video.mp4");
        bash.push("rm audio.mp4");
        bash.push("rm genVOD.sh");

        var a = document.createElement("a");
        a.href = URL.createObjectURL(new Blob([bash.join("\n")], {type: "text/x-shellscript"}));
        a.download = "genVOD.sh";
        a.click();
    });
    xhr.send();
}

// genBash("master.json?base64_init=1");

/*

Instructions

1. Open Developer Tools and switch to the Network tab.
2. Play the video to start loading video files. You may pause immediately.
3. Search the Network tab for `master.json`. Copy the URL.
4. Run `genBash(URL)` to download `genVOD.sh`.
5. In the Terminal, navigate to the folder with the downloaded bash file.
6. Run `bash genVOD.sh` to download `vod.mp4`.

Notes

- This is based on Vimeo-hosted videos; specifically, for Moment House.
- The best files are determined via bitrate.
- This script should work if the best files' segments are M4S files.
- If the best files' segments are MP4 files, just download the MP4 without the query parameters.
  - Edit the genVOD.sh file to skip the corresponding segment downloads and to adjust merging.

*/

master_example.json

{
    "clip_id": "a0b1c2d3-e4f5-g6h7-i8j9-k0l1m2n3o4p5",
    "base_url": "../../../",
    "video": [
        {
            "id": "q6r7s8t9",
            "base_url": "q6r7s8t9/chop/",
            "format": "mp42",
            "mime_type": "video/mp4",
            "codecs": "avc1.640028",
            "bitrate": 5849000,
            "avg_bitrate": 4324000,
            "duration": 24,
            "framerate": 30,
            "width": 1920,
            "height": 1080,
            "max_segment_duration": 6,
            "init_segment": "dmlkZW8=",
            "segments": [
                {"start": 0, "end": 6, "url": "segment-1.m4s", "size": 1933385},
                {"start": 6, "end": 12, "url": "segment-2.m4s", "size": 2387826},
                {"start": 12, "end": 18, "url": "segment-3.m4s", "size": 1369615},
                {"start": 18, "end": 24, "url": "segment-4.m4s", "size": 2065895}
            ]
        }
    ],
    "audio": [
        {
            "id": "u0v1w2x3",
            "base_url": "../audio/u0v1w2x3/chop/",
            "format": "mp42",
            "mime_type": "audio/mp4",
            "codecs": "mp4a.40.2",
            "bitrate": 255000,
            "avg_bitrate": 255000,
            "duration": 24,
            "channels": 2,
            "sample_rate": 48000,
            "max_segment_duration": 6,
            "init_segment": "YXVkaW8=",
            "segments": [
                {"start": 0, "end": 6, "url": "segment-1.m4s", "size": 193189},
                {"start": 6, "end": 12, "url": "segment-2.m4s", "size": 193869},
                {"start": 12, "end": 18, "url": "segment-3.m4s", "size": 192506},
                {"start": 18, "end": 24, "url": "segment-4.m4s", "size": 193189}
            ]
        }
    ]
}

Found a much easier method a day after writing this: https://gist.github.com/alexeygrigorev/a1bc540925054b71e1a7268e50ad55cd#gistcomment-3579183