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Merge Matroska and WebM files using Native Messaging, mkvmerge, JavaScript

Tested at Chromium 77, which currently supports Native File System for writing files to local filesystem using JavaScript shipped with the browser. mkvmerge requires the complete file at local filesystem. TODO: Write version for Firefox.


  • Download the repository.
  • Build or install MKVToolNix version 36.0.0.
  • Save mkvmerge in host folder and set cmd as "./mkvmerge" or "mkvmerge" to execute the command installed by package manager.
  • Open native_messaging_mkvmerge.json within host folder in a text editor. Set value of "path" to local path to native-messaging-host.js.
  • Open and within host folder in a text editor. Set the value of TARGET_DIR within and to point to NativeMessagingHost folder within Chromium user data directory.
  • Set mkvmerge, .js and .sh files executable.
  • Install nodejs.
  • Run ./
  • At chrome://extensions set Developer mode to on.
  • Click Load unpacked.
  • Select app folder.


  • Click Apps on Chromium browser bookmarks bar.
  • Click native-messaging-mkvmerge icon.
  • Input URL's to media files the <video> element can play, optionally including a media fragment identifier, to fetch delimited by space character at <textarea> element, e.g., <textarea>https://path/to/0.ogg#t=0,2 https://path/to/1.webm#t=0,2 https://path/to/2.mp4 https:/path/to/3.ogv </textarea> to be played and recorded using MediaRecorder.
  • Optionally upload files from host folder to <input type="file" multiple>.
  • Click Connect.
  • Click Send.
  • Grant permission for "Let sites read this folder" prompt
  • Grant permission for "Save changes to original files" prompt

Output: A <video> element will be appended to the HTML document with src to a Blob URL representation of the merged media files as a Blob.

Note: If the resulting Blob does not display all of the merged videos the first time Connect and Send are clicked, navigate to chrome://extensions select the extension, click the reload symbol to the right of Details and Remove, then repeat the above process.


Navigate to chrome://extensions, select the extenion, click Remove. Run ./

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