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webm-muxer - JavaScript WebM multiplexer

The WebCodecs API provides low-level access to media codecs, but provides no way of actually packaging (multiplexing) the encoded media into a playable file. This project implements a WebM/Matroska multiplexer in pure TypeScript, which is high-quality, fast and tiny, and supports video, audio and subtitles as well as live-streaming.

Demo: Muxing into a file

Demo: Streaming

Note: If you're looking to create MP4 files, check out mp4-muxer, the sister library to webm-muxer.

Quick start

The following is an example for a common usage of this library:

import { Muxer, ArrayBufferTarget } from 'webm-muxer';

let muxer = new Muxer({
    target: new ArrayBufferTarget(),
    video: {
        codec: 'V_VP9',
        width: 1280,
        height: 720

let videoEncoder = new VideoEncoder({
    output: (chunk, meta) => muxer.addVideoChunk(chunk, meta),
    error: e => console.error(e)
    codec: 'vp09.00.10.08',
    width: 1280,
    height: 720,
    bitrate: 1e6

/* Encode some frames... */

await videoEncoder.flush();

let { buffer } =; // Buffer contains final WebM file


This library was created to power the in-game video renderer of the browser game Marble Blast Web - here you can find a video completely rendered by it and muxed with this library. Previous efforts at in-browser WebM muxing, such as webm-writer-js or webm-muxer.js, were either lacking in functionality or were way too heavy in terms of byte size, which prompted the creation of this library.


Using NPM, simply install this package using

npm install webm-muxer

You can import all exported classes like so:

import * as WebMMuxer from 'webm-muxer';
// Or, using CommonJS:
const WebMMuxer = require('webm-muxer');

Alternatively, you can simply include the library as a script in your HTML, which will add a WebMMuxer object, containing all the exported classes, to the global object, like so:

<script src="build/webm-muxer.js"></script>



For each WebM file you wish to create, create an instance of Muxer like so:

import { Muxer } from 'webm-muxer';

let muxer = new Muxer(options);

The available options are defined by the following interface:

interface MuxerOptions {
        | ArrayBufferTargetStreamTargetFileSystemWritableFileStreamTarget,

    video?: {
        codec: string,
        width: number,
        height: number,
        frameRate?: number, // Optional, adds metadata to the file
        alpha?: boolean // If the video contains transparency data

    audio?: {
        codec: string,
        numberOfChannels: number,
        sampleRate: number,
        bitDepth?: number // Mainly necessary for PCM-coded audio

    subtitles?: {
        codec: string

    streaming?: boolean,

    type?: 'webm''matroska',

    firstTimestampBehavior?: 'strict' | 'offset' | 'permissive'

Codecs officially supported by WebM are:
Video: V_VP8, V_VP9, V_AV1
Subtitles: S_TEXT/WEBVTT


This option specifies where the data created by the muxer will be written. The options are:

  • ArrayBufferTarget: The file data will be written into a single large buffer, which is then stored in the target.

    import { Muxer, ArrayBufferTarget } from 'webm-muxer';
    let muxer = new Muxer({
        target: new ArrayBufferTarget(),
        // ...
    // ...
    let { buffer } =;
  • StreamTarget: This target defines callbacks that will get called whenever there is new data available - this is useful if you want to stream the data, e.g. pipe it somewhere else. The constructor has the following signature:

    constructor(options: {
        onData?: (data: Uint8Array, position: number) => void,
        onHeader?: (data: Uint8Array, position: number) => void,
        onCluster?: (data: Uint8Array, position: number, timestamp: number) => void,
        chunked?: boolean,
        chunkSize?: number

    onData is called for each new chunk of available data. The position argument specifies the offset in bytes at which the data has to be written. Since the data written by the muxer is not entirely sequential, make sure to respect this argument.

    When using chunked: true, data created by the muxer will first be accumulated and only written out once it has reached sufficient size. This is useful for reducing the total amount of writes, at the cost of latency. It using a default chunk size of 16 MiB, which can be overridden by manually setting chunkSize to the desired byte length.

    If you want to use this target for live-streaming, make sure to also set streaming: true in the muxer options. This will ensure that data is written monotonically (sequentially) and already-written data is never "patched" - necessary for live-streaming, but not recommended for muxing files for later viewing.

    The onHeader and onCluster callbacks will be called for the file header and each Matroska cluster, respectively. This way, you don't need to parse them out yourself from the data provided by onData.

    import { Muxer, StreamTarget } from 'webm-muxer';
    let muxer = new Muxer({
        target: new StreamTarget({
            onData: (data, position) => { /* Do something with the data */ }
        // ...
  • FileSystemWritableFileStreamTarget: This is essentially a wrapper around a chunked StreamTarget with the intention of simplifying the use of this library with the File System Access API. Writing the file directly to disk as it's being created comes with many benefits, such as creating files way larger than the available RAM.

    You can optionally override the default chunkSize of 16 MiB.

        stream: FileSystemWritableFileStream,
        options?: { chunkSize?: number }

    Usage example:

    import { Muxer, FileSystemWritableFileStreamTarget } from 'webm-muxer';
    let fileHandle = await window.showSaveFilePicker({
        suggestedName: `video.webm`,
        types: [{
            description: 'Video File',
            accept: { 'video/webm': ['.webm'] }
    let fileStream = await fileHandle.createWritable();
    let muxer = new Muxer({
        target: new FileSystemWritableFileStreamTarget(fileStream),
        // ...
    // ...
    await fileStream.close(); // Make sure to close the stream
streaming (optional)

Configures the muxer to only write data monotonically, useful for live-streaming the WebM as it's being muxed; intended to be used together with the target set to type function. When enabled, some features such as storing duration and seeking will be disabled or impacted, so don't use this option when you want to write out WebM file for later use.

type (optional)

As WebM is a subset of the more general Matroska multimedia container format, this library muxes both WebM and Matroska files. WebM, according to the official specification, supports only a small subset of the codecs supported by Matroska. It is likely, however, that most players will successfully play back a WebM file with codecs other than the ones supported in the spec. To be on the safe side, however, you can set the type option to 'matroska', which will internally label the file as a general Matroska file. If you do this, your output file should also have the .mkv extension.

firstTimestampBehavior (optional)

Specifies how to deal with the first chunk in each track having a non-zero timestamp. In the default strict mode, timestamps must start with 0 to ensure proper playback. However, when directly pumping video frames or audio data from a MediaTrackStream into the encoder and then the muxer, the timestamps are usually relative to the age of the document or the computer's clock, which is typically not what we want. Handling of these timestamps must be set explicitly:

  • Use 'offset' to offset the timestamp of each video track by that track's first chunk's timestamp. This way, it starts at 0.
  • Use 'permissive' to allow the first timestamp to be non-zero.

Muxing media chunks

Then, with VideoEncoder and AudioEncoder set up, send encoded chunks to the muxer using the following methods:

    chunk: EncodedVideoChunk,
    meta?: EncodedVideoChunkMetadata,
    timestamp?: number
): void;

    chunk: EncodedAudioChunk,
    meta?: EncodedAudioChunkMetadata,
    timestamp?: number
): void;

Both methods accept an optional, third argument timestamp (microseconds) which, if specified, overrides the timestamp property of the passed-in chunk.

The metadata comes from the second parameter of the output callback given to the VideoEncoder or AudioEncoder's constructor and needs to be passed into the muxer, like so:

let videoEncoder = new VideoEncoder({
    output: (chunk, meta) => muxer.addVideoChunk(chunk, meta),
    error: e => console.error(e)
videoEncoder.configure(/* ... */);

Should you have obtained your encoded media data from a source other than the WebCodecs API, you can use these following methods to directly send your raw data to the muxer:

    data: Uint8Array,
    type: 'key' | 'delta',
    timestamp: number, // In microseconds
    meta?: EncodedVideoChunkMetadata
): void;

    data: Uint8Array,
    type: 'key' | 'delta',
    timestamp: number, // In microseconds
    meta?: EncodedAudioChunkMetadata
): void;

Finishing up

When encoding is finished and all the encoders have been flushed, call finalize on the Muxer instance to finalize the WebM file:


When using an ArrayBufferTarget, the final buffer will be accessible through it:

let { buffer } =;

When using a FileSystemWritableFileStreamTarget, make sure to close the stream after calling finalize:

await fileStream.close();


Video key frame frequency

Canonical WebM files can only have a maximum Matroska Cluster length of 32.768 seconds, and each cluster must begin with a video key frame. You therefore need to tell your VideoEncoder to encode a VideoFrame as a key frame at least every 32 seconds, otherwise your WebM file will be incorrect. You can do this by doing:

videoEncoder.encode(frame, { keyFrame: true });

Media chunk buffering

When muxing a file with a video and an audio track, it is important that the individual chunks inside the WebM file be stored in monotonically increasing time. This does mean, however, that the multiplexer must buffer chunks of one medium if the other medium has not yet encoded chunks up to that timestamp. For example, should you first encode all your video frames and then encode the audio afterwards, the multiplexer will have to hold all those video frames in memory until the audio chunks start coming in. This might lead to memory exhaustion should your video be very long. When there is only one media track, this issue does not arise. So, when muxing a multimedia file, make sure it is somewhat limited in size or the chunks are encoded in a somewhat interleaved way (like is the case for live media).


This library supports adding a subtitle track to a file. Like video and audio, subtitles also need to be encoded before they can be added to the muxer. To do this, this library exports its own SubtitleEncoder class with a WebCodecs-like API. Currently, it only supports encoding WebVTT files.

Here's a full example using subtitles:

import { Muxer, SubtitleEncoder, ArrayBufferTarget } from 'webm-muxer';

let muxer = new Muxer({
    target: new ArrayBufferTarget(),
    subtitles: {
        codec: 'S_TEXT/WEBVTT'
    // ....

let subtitleEncoder = new SubtitleEncoder({
    output: (chunk, meta) => muxer.addSubtitleChunk(chunk, meta),
    error: e => console.error(e)
    codec: 'webvtt'

let simpleWebvttFile =

00:00:00.000 --> 00:00:10.000
Example entry 1: Hello <b>world</b>.

// ...


You do not need to encode an entire WebVTT file in one go; you can encode individual cues or any number of them at once. Just make sure that the preamble (the part before the first cue) is the first thing to be encoded.

Size "limits"

This library can mux WebM files up to a total size of ~4398 GB and with a Matroska Cluster size of ~34 GB.

Implementation & development

WebM files are a subset of the more general Matroska media container format. Matroska in turn uses a format known as EBML (think of it like binary XML) to structure its file. This project therefore implements a simple EBML writer to create the Matroska elements needed to form a WebM file. Many thanks to webm-writer-js for being the inspiration for most of the core EBML writing code.

For development, clone this repository, install everything with npm install, then run npm run watch to bundle the code into the build directory. Run npm run check to run the TypeScript type checker, and npm run lint to run ESLint.



Last updated on 08 Jan 2024

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