ffmpeg.js converting webm from a media recorder into mp4

wat.js

  var ffmpeg = require('ffmpeg.js/ffmpeg-mp4.js')

  document.querySelector('button').addEventListener('click', (evt) => {
    document.querySelector('[camera]').setAttribute('animation', {
      property: 'rotation',
      to: '0 360 0',
      dur: 10000,
      easing: 'linear'
    })

    var stream = document.querySelector('canvas').captureStream(25)
    var recorder = new MediaRecorder(stream, {mimeType: 'video/webm;codecs=h264'})
    var frames = []
    console.log(recorder.mimeType)
    recorder.ondataavailable = function(evt) {
      if(evt.data) frames.push(evt.data)
    }

    recorder.onstop = function() {
      var fileBlob = new Blob(frames, { type: 'video/webm;codecs=h264' })

      var reader = new FileReader()
      reader.onload = function(evt) {
        var mp4 = ffmpeg({
          MEMFS: [{name: "test.webm", data: evt.target.result}],
          arguments: ['-i', 'test.webm','-vcodec', 'copy', '-qscale', '0', 'test.mp4'],
          stdin: function() {}
        })

        var mp4blob = new Blob([mp4.MEMFS[0].data], {type: 'video/mp4'})
        console.log(mp4blob)
        var dataUrl = window.URL.createObjectURL(mp4blob)
        var link = document.createElement('a')
        link.href = dataUrl
        link.download = 'recording-ftw.mp4'
        link.click()

      }
      reader.readAsArrayBuffer(fileBlob)

      var dataUrl = window.URL.createObjectURL(fileBlob)
      var link = document.createElement('a')
      link.href = dataUrl
      link.download = 'recording.webm'
      link.click()

    }

    recorder.start(100)
    setTimeout(() => {
      recorder.stop()
    }, 10000)
  })

where is "ffmpeg.js/ffmpeg-mp4.js"?

Oof, this gist is 7 years old...

If I remember and this Google search hasn't misled me - it's this https://github.com/Kagami/ffmpeg.js/

May the odds ever be in your favour.