A MediaRecorder ponyfill that records audio as mp3. It uses the great Kagami/vmsg library under the hood to encode mp3 audio in WebAssembly using LAME.

View the live demo

• Standard MediaRecorder API
• Audio encoding off the main thread using Web Workers
• Consistent MP3 file output in all supported browsers
• High quality type definitions
• 9kB main library
• 80kB Web Worker with WebAssembly module (Loaded async)

• Chrome 57+
• Firefox 52+
• Safari 11+
• Edge 16+

Install with npm or yarn.

yarn add mp3-mediarecorder

If you don't want to set up a build environment, you can get mp3-mediarecorder from a CDN like unpkg.com and it will be globally available through the window.mp3MediaRecorder object.

<script src="https://unpkg.com/mp3-mediarecorder"></script>

We'll have two files: index.js and worker.js. The first is what we import from our app, so it runs on the main thread — it imports our worker (using worker-loader or workerize-loader) and passes it to Mp3MediaRecorder to create a recorder instance around it.

import { Mp3MediaRecorder } from 'mp3-mediarecorder';
import Mp3RecorderWorker from 'workerize-loader!./worker';

const recorder = new Mp3MediaRecorder(
    mediaStream, // MediaStream instance
    { worker: Mp3RecorderWorker() },
);
recorder.start(); // 🎉

In most cases the MediaStream instance will come from the getUserMedia API. For a usage example, see here.

import { initMp3MediaEncoder } from 'mp3-mediarecorder/worker';

initMp3MediaEncoder({ vmsgWasmUrl: '/url/to/vmsg.wasm' });

The second file is our worker code, which runs in the background thread. Here we import initMp3MediaEncoder from mp3-mediarecorder/worker. This sets things up to communicate with the main thread.

Mp3MediaRecorder is a class that has the same API as the standard MediaRecorder. If you want to see the full API please check out the documentation on MDN.

Constructor parameters

The Mp3MediaRecorder constructor parameters differ from the standard API.

• mediaStream: MediaStream An instance of MediaStream (eg: from getUserMedia)

• options: Mp3MediaRecorderOptions

  • worker: Worker An instantiated Web Worker (eg: new Worker('./worker.js'))
  • audioContext?: AudioContextAn instantiated AudioContext (eg: new AudioContext()) This might be useful if you want to full control over the AudioContext. Chrome and Safari limit the number of AudioContext objects.

Example

const recorder = new Mp3MediaRecorder(
    mediaStream, // MediaStream instance
    {
        worker: Mp3RecorderWorker(),
        // Optionally supply your own AudioContext
        audioContext: new AudioContext(),
    },
);

The Web Worker side the of the recorder. The worker will communicate with the main thread to encode the mp3 file.

Sets up the communication with the main thread.

Parameters

• vmsgWasmUrl: string The URL of the vmsg.wasm file. This could be self-hosted or from a CDN. The Worker fill fetch this URL and instantiate a WebAssembly module from it.

Example

import { initMp3MediaEncoder } from 'mp3-mediarecorder/worker';

initMp3MediaEncoder({ vmsgWasmUrl: '/url/to/vmsg.wasm' });

Browser support for MediaRecorder is lacking.

Even in browsers with support for MediaRecorder, the available audio formats differ between browsers, and are not always compatible with other browsers. MP3 is the only audio format that can be played by all modern browsers.

Kagami/vmsg is a great library but I needed something that doesn't include a UI and/or getUserMedia code.

• In Safari, pause and resume does not work (see #60)
• The dataavailable event only fires once, when encoding is complete. MediaRecorder.start ignores its optional timeSlice argument. As a result,MediaRecorder.requestData does not trigger a dataavailable event
• bitsPerSecond is not configurable, the MediaRecorder constructor will ignore this option.

yarn dev

A development version of the demo will be served on http://localhost:1234.

• Kagami/vmsg: Use this library if you want a more complete microphone recording library with a built-in UI