MediaSource hijack.js

(() => {

  const insertScript = src => {
    const elem = document.createElement('script')
    elem.src = src
    document.head.append(elem)
  }

  const proxy = (original, proxyFn) => function (...args) {
    try {
      return proxyFn(original, this, args)
    } catch (e) {
      console.error('Error in proxy!', e)
      throw e
    }
  }

  const patchPrototype = (Constr, patch) => {
    // doesnt support getters & setters
    const proto = Constr.prototype
    Object.entries(patch)
      .map(([k, v]) => [k, proxy(proto[k], v)])
      .forEach(([k, v]) => proto[k] = v)
  }

  insertScript('https://cdn.jsdelivr.net/npm/[email protected]/dist/ponyfill.min.js')
  insertScript('https://cdn.jsdelivr.net/npm/[email protected]/StreamSaver.min.js')

  const writer = new Promise(res => setTimeout(async () => {
    window.streamSaver.WritableStream = window.WritableStream || window.WebStreamsPolyfill.WritableStream
    const stream = window.streamSaver.createWriteStream('test.mp4')
    const writer = stream.getWriter()
    window.__writer = writer //debug
    await writer.ready
    res(writer)
  }, 100))

  patchPrototype(window.MediaSource, {
    addSourceBuffer: (original, thisArg, args) => {
      const id = thisArg.sourceBuffers.length
      console.log(`New source buffer (id ${id}): ${args[0]}`)
      const result = original.apply(thisArg, args)
      result._id = id
      result._mime = args[0]
      return result
    }
  })

  patchPrototype(window.SourceBuffer, {
    appendBuffer: (original, thisArg, args) => {
      console.log(`[${thisArg._id}/'${thisArg._mime}'] Append buffer`, args)
      if (thisArg._mime.includes('video')) {
        writer.then(handle => handle.write(args[0]))
      }
      return original.apply(thisArg, args)
    }
  })
})()