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0.8.3 • Public • Published


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opus-media-recorder is a MediaRecorder API polyfill written in ES6 and WebAssembly. It aims a cross-browser Opus codec support with various audio formats such as Ogg and WebM. opus-media-recorder can be used as a ployfill, or it can replace the built-in MediaRecorder since opus-media-recorder supports more MIME types.

opus-media-recorder uses WebAssembly compiled from popular libraries (e.g libopus, libogg, libwebm, and speexdsp) to ensure performance and standards-compliance.

Why opus-media-recorder?

opus-media-recorder Chrome Firefox iOS Edge
audio/ogg O X O X X
audio/webm O O X X X
audio/wav O X X X X

* Both audio/ogg and audio/webm refer containers for Opus audio codec.

Currently the MediaRecorder API suffers from the two problems:

  1. Not all browsers support MediaRecorder.
  2. Even the browsers that provides MediaRecorder don't support the same format.

opus-media-recorder tackles these problems by supporting all major modern browsers (Chrome, Firefox, iOS, and Edge) and by providing various formats.

By taking advantages of WebAssembly and Web Workers, opus-media-recorder tries to have minimum performace panalties of running encoders on a browser.

How to use

opus-media-recorder is compatible with the Mediastream Recording API standard.

Thing to know

  • The page must be served over HTTPS in order to record.
  • Being able to record does not always mean you can play it on browsers:
    • macOS/iOS Safari cannot play Opus natively yet.
    • Old Edge requires an extension to play Opus natively.
    • You can get an Opus decorder to play it. There are Opus decoders available, such as Chris Rudmin's Opus decoder.
    • Otherwise, users can download as a file and play it using apps like VLC.
  • When mimeType is not specified a default encoder is loading depending on OS:
    • Chrome: audio/webm
    • Firefox: audio/ogg
    • Edge: audio/webm
    • iOS/macOS Safari: audio/wave - because they cannot play Opus at all.


For standard usages of MediaRecorder, see the MDN reference and other online resources. Our testing website and example section may be useful as well.


npm install --save-dev opus-media-recorder

Working with a bundler

Because opus-media-recorder needs to load a dedicated web worker and .wasm binaries, special configurations are necessary. In general:

import OpusMediaRecorder from 'opus-media-recorder';
// Choose desired format like audio/webm. Default is audio/ogg
const options = { mimeType: 'audio/ogg' }
// Web worker and .wasm configuration. Note: This is NOT a part of W3C standard.
const workerOptions = {
  encoderWorkerFactory: function () {
    return new Worker('.../path/to/opus-media-recorder/encoderWorker.js')
  OggOpusEncoderWasmPath: '.../path/to/opus-media-recorder/OggOpusEncoder.wasm',
  WebMOpusEncoderWasmPath: '.../path/to/opus-media-recorder/WebMOpusEncoder.wasm'
window.MediaRecorder = OpusMediaRecorder;
recorder = new MediaRecorder(stream, options, workerOptions);

Bundler-specific examples:

Simple JavaScript example (webpack)

import OpusMediaRecorder from 'opus-media-recorder';
// Use worker-loader
import Worker from 'worker-loader!opus-media-recorder/encoderWorker.js';
// You should use file-loader in webpack.config.js.
// See webpack example link in the above section for more detail.
import OggOpusWasm from 'opus-media-recorder/OggOpusEncoder.wasm';
import WebMOpusWasm from 'opus-media-recorder/WebMOpusEncoder.wasm';
// Non-standard options
const workerOptions = {
  encoderWorkerFactory: _ => new Worker(),
  OggOpusEncoderWasmPath: OggOpusWasm,
  WebMOpusEncoderWasmPath: WebMOpusWasm
window.MediaRecorder = OpusMediaRecorder;
let recorder;
function startRecording () {
  navigator.mediaDevices.getUserMedia({ audio: true }).then(stream => {
    let options = { mimeType: 'audio/ogg' };
    // Start recording
    recorder = new MediaRecorder(stream, options, workerOptions);
    // Set record to <audio> when recording will be finished
    recorder.addEventListener('dataavailable', (e) => {
      audioElement.src = URL.createObjectURL(e.data);
// Recording should be started in user-initiated event like buttons
recordButton.addEventListener('click', startRecording);
// Stop recording
stopButton.addEventListener('click', () => {
  // Remove “recording” icon from browser tab
  recorder.stream.getTracks().forEach(i => i.stop());

HTML <script> tag

The OpusMediaRecorder object is available in the global namespace using UMD.

<!-- load OpusMediaRecorder.umd.js. OpusMediaRecorder will be loaded. -->
<script src="https://cdn.jsdelivr.net/npm/opus-media-recorder@latest/OpusMediaRecorder.umd.js"></script>
<!-- load encoderWorker.umd.js. This should be after OpusMediaRecorder. -->
<!-- OpusMediaRecorder.encoderWorker will be loaded. -->
<script src="https://cdn.jsdelivr.net/npm/opus-media-recorder@latest/encoderWorker.umd.js"></script>
// If you loaded encoderWorker.js using <script> tag,
// you don't need to define encoderWorkerFactory.
const workerOptions = {
  OggOpusEncoderWasmPath: 'https://cdn.jsdelivr.net/npm/opus-media-recorder@latest/OggOpusEncoder.wasm',
  WebMOpusEncoderWasmPath: 'https://cdn.jsdelivr.net/npm/opus-media-recorder@latest/WebMOpusEncoder.wasm'
// Existing MediaRecorder is replaced
window.MediaRecorder = OpusMediaRecorder;
let recorder = new MediaRecorder(stream, {}, workerOptions);

Use opus-media-recorder only when a browser doesn't support it

// Check if MediaRecorder available.
if (!window.MediaRecorder) {
  window.MediaRecorder = OpusMediaRecorder;
// Check if a target format (e.g. audio/ogg) is supported.
else if (!window.MediaRecorder.isTypeSupported('audio/ogg;codecs=opus')) {
  window.MediaRecorder = OpusMediaRecorder;

Browser support


  • Chrome >= 58
  • Firefox >= 53
  • Microsoft Edge >= 41
  • Safari (macOS and iOS) >= 11

Browsers with issues:

MIME Type support

  • audio/webm
  • audio/webm; codecs=opus
  • audio/ogg
  • audio/ogg; codecs=opus
  • audio/wav or audio/wave


  • opus-media-recorder throws generic Error objects instead of native DOMException.
  • Because audio/wav is not designed for streaming, when mimeType is audio/wav, each dataavailabe events produces a complete and separated .wav file that cannot be concatenated together unlike Ogg and WebM. Therefore, it is recommended not to use timeslice option when calling start(), unless you know what the implication is.
  • There is no SecurityError case implemented. (WIP)

How to build

  1. To build from the source, you need Emscripten version 1.38.25, NPM, npx, Python 2.7 or higher, and basic C program build systems such as GNU Make. Environment variable $EMSCRIPEN must be set in order to build.

  2. npm install to install JavaScript dependencies.

  3. npm run build to build. npm run build:production to build files for distribution.

  4. npm run serve to run a test web server locally. Default URL is https://localhost:9000 (It has to be HTTPS). You might have to change the src path of <script> at the bottom of docs/index.html to test it correctly (I'm finding a way to automate this.)

  5. npm run clean to clean up build files.


npm i webm-media-recorder

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