TypeScript icon, indicating that this package has built-in type declarations

3.0.2 • Public • Published

mp4-muxer - JavaScript MP4 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 an MP4 multiplexer in pure TypeScript which is high-quality, fast and tiny, and supports both video and audio.

Demo: Muxing into a file

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

Consider donating if you've found this library useful and wish to support it ❤️

Quick start

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

import { Muxer, ArrayBufferTarget } from 'mp4-muxer';

let muxer = new Muxer({
    target: new ArrayBufferTarget(),
    video: {
        codec: 'avc',
        width: 1280,
        height: 720
    fastStart: 'in-memory'

let videoEncoder = new VideoEncoder({
    output: (chunk, meta) => muxer.addVideoChunk(chunk, meta),
    error: e => console.error(e)
    codec: 'avc1.42001f',
    width: 1280,
    height: 720,
    bitrate: 1e6

/* Encode some frames... */

await videoEncoder.flush();

let { buffer } =; // Buffer contains final MP4 file


After webm-muxer gained traction for its ease of use and integration with the WebCodecs API, this library was created to now also allow the creation of MP4 files while maintaining the same DX. While WebM is a more modern format, MP4 is an established standard and supported on way more devices.


Using NPM, simply install this package using

npm install mp4-muxer

You can import all exported classes like so:

import * as Mp4Muxer from 'mp4-muxer';
// Or, using CommonJS:
const Mp4Muxer = require('mp4-muxer');

Alternatively, you can simply include the library as a script in your HTML, which will add an Mp4Muxer object, containing all the exported classes, to the global object, like so:

<script src="build/mp4-muxer.js"></script>



For each MP4 file you wish to create, create an instance of Muxer like so:

import { Muxer } from 'mp4-muxer';

let muxer = new Muxer(options);

The available options are defined by the following interface:

interface MuxerOptions {
        | ArrayBufferTarget
        | StreamTarget
        | FileSystemWritableFileStreamTarget,

    video?: {
        codec: 'avc' | 'hevc' | 'vp9' | 'av1',
        width: number,
        height: number,
        rotation?: 0 | 90 | 180 | 270 // Adds rotation metadata to the file

    audio?: {
        codec: 'aac' | 'opus',
        numberOfChannels: number,
        sampleRate: number

        | false
        | 'in-memory'
        | { expectedVideoChunks?: number, expectedAudioChunks?: number }

    firstTimestampBehavior?: 'strict' | 'offset'

Codecs currently supported by this library are AVC/H.264, HEVC/H.265, VP9 and AV1 for video, and AAC and Opus for audio.

target (required)

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 'mp4-muxer';
    let muxer = new Muxer({
        target: new ArrayBufferTarget(),
        fastStart: 'in-memory',
        // ...
    // ...
    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:

        onData: (data: Uint8Array, position: number) => void,
        onDone?: () => void,
        options?: { chunked?: true, chunkSize?: number }

    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 in the options, 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.

    Note that this target is not intended for live-streaming, i.e. playback before muxing has finished.

    import { Muxer, StreamTarget } from 'mp4-muxer';
    let muxer = new Muxer({
        target: new StreamTarget(
            (data, position) => { /* Do something with the data */ },
            () => { /* Muxing has finished */ }
        fastStart: false,
        // ...
  • 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 'mp4-muxer';
    let fileHandle = await window.showSaveFilePicker({
        suggestedName: `video.mp4`,
        types: [{
            description: 'Video File',
            accept: { 'video/mp4': ['.mp4'] }
    let fileStream = await fileHandle.createWritable();
    let muxer = new Muxer({
        target: new FileSystemWritableFileStreamTarget(fileStream),
        fastStart: false,
        // ...
    // ...
    await fileStream.close(); // Make sure to close the stream

fastStart (required)

By default, MP4 metadata is stored at the end of the file in the moov box - this makes writing the file faster and easier. However, placing this moov box at the start of the file instead (known as "Fast Start") provides certain benefits: The file becomes easier to stream over the web without range requests, and sites like YouTube can start processing the video while it's uploading. This library provides full control over the placement of the moov box by setting fastStart to one of these options:

  • false: Disables Fast Start, placing metadata at the end of the file. This option is the fastest and uses the least memory. This option is recommended for large, unbounded files that are streamed directly to disk.

  • 'in-memory': Produces a file with Fast Start by keeping all media chunks in memory until the file is finalized. This option produces the most compact output possible at the cost of a more expensive finalization step and higher memory requirements. You should always use this option when using ArrayBufferTarget as it will result in a higher-quality output with no change in memory footprint.

  • object: Produces a file with Fast Start by reserving space for metadata when muxing begins. To know how many bytes need to be reserved to be safe, you'll have to provide the following data:

        expectedVideoChunks?: number,
        expectedAudioChunks?: number

    Note that the property expectedVideoChunks is required if you have a video track - the same goes for audio. With this option set, you cannot mux more chunks than the number you've specified (although less is fine).

    This option is faster than 'in-memory' and uses no additional memory, but results in a slightly larger output, making it useful for when you want to stream the file to disk while still retaining Fast Start.

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 piping 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.

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
    duration: number, // in microseconds
    meta?: EncodedVideoChunkMetadata
): void;

    data: Uint8Array,
    type: 'key' | 'delta',
    timestamp: number, // in microseconds
    duration: 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 MP4 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();


Variable frame rate

MP4 files support variable frame rate, however some players (such as QuickTime) have been observed not to behave well when the timestamps are irregular. Therefore, whenever possible, try aiming for a fixed frame rate.

Implementation & development

MP4 files are based on the ISO Base Media Format, which structures its files as a hierarchy of boxes (or atoms). The standards used to implement this library were ISO/IEC 14496-1, ISO/IEC 14496-12 and ISO/IEC 14496-14. Additionally, the QuickTime MP4 Specification was a very useful resource.

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.

Package Sidebar


npm i mp4-muxer

Weekly Downloads






Unpacked Size

111 kB

Total Files


Last publish


  • vanilagy