Output

Create and write output media files

The Output class orchestrates the creation of a new media file, allowing you to add tracks and configure output settings.

Constructor

new Output(options: OutputOptions)

Creates a new instance of Output which can then be used to create a new media file according to the specified OutputOptions.

options · OutputOptions · required

Configuration for the output file

Show OutputOptions properties

format · OutputFormat · required

The format of the output file.

target · Target · required

The target to which the file will be written.

Properties

format · OutputFormat

The format of the output file.

target · Target

The target to which the file will be written.

state · 'pending' | 'started' | 'canceled' | 'finalizing' | 'finalized'

The current state of the output.

Methods

addVideoTrack()

addVideoTrack(source: VideoSource, metadata?: VideoTrackMetadata): void

Adds a video track to the output with the given source. Can only be called before the output is started.

source · VideoSource · required

The video source for this track.

metadata · VideoTrackMetadata

Track metadata including language, name, rotation, and frame rate

Show VideoTrackMetadata properties

languageCode · string

The three-letter, ISO 639-2/T language code specifying the language of this track.

name · string

A user-defined name for this track, like “English” or “Director Commentary”.

disposition · Partial<TrackDisposition>

The track’s disposition, i.e. information about its intended usage.

rotation · Rotation

The angle in degrees by which the track’s frames should be rotated (clockwise). Must be 0, 90, 180, or 270.

frameRate · number

The expected video frame rate in hertz. If set, all timestamps and durations of this track will be snapped to this frame rate.

maximumPacketCount · number

The maximum amount of encoded packets that will be added to this track. Setting this field provides the muxer with an additional signal that it can use to preallocate space in the file.

addAudioTrack()

addAudioTrack(source: AudioSource, metadata?: AudioTrackMetadata): void

Adds an audio track to the output with the given source. Can only be called before the output is started.

source · AudioSource · required

The audio source for this track.

metadata · AudioTrackMetadata

Track metadata including language, name, and disposition

Show AudioTrackMetadata properties

languageCode · string

The three-letter, ISO 639-2/T language code specifying the language of this track.

name · string

A user-defined name for this track.

disposition · Partial<TrackDisposition>

The track’s disposition, i.e. information about its intended usage.

maximumPacketCount · number

The maximum amount of encoded packets that will be added to this track.

addSubtitleTrack()

addSubtitleTrack(source: SubtitleSource, metadata?: SubtitleTrackMetadata): void

Adds a subtitle track to the output with the given source. Can only be called before the output is started.

source · SubtitleSource · required

The subtitle source for this track.

metadata · SubtitleTrackMetadata

Track metadata including language, name, and disposition.

setMetadataTags()

setMetadataTags(tags: MetadataTags): void

Sets descriptive metadata tags about the media file, such as title, author, date, or cover art. When called multiple times, only the metadata from the last call will be used.

Can only be called before the output is started.

tags · MetadataTags · required

Metadata tags to write to the output file.

start()

async start(): Promise<void>

Starts the creation of the output file. This method should be called after all tracks have been added. Only after the output has started can media samples be added to the tracks.

Returns a promise that resolves when the output has successfully started and is ready to receive media samples.

getMimeType()

async getMimeType(): Promise<string>

Resolves with the full MIME type of the output file, including track codecs.

The returned promise will resolve only once the precise codec strings of all tracks are known.

finalize()

async finalize(): Promise<void>

Finalizes the output file. This method must be called after all media samples across all tracks have been added. Once the Promise returned by this method completes, the output file is ready.

cancel()

async cancel(): Promise<void>

Cancels the creation of the output file, releasing internal resources like encoders and preventing further samples from being added.

Returns a promise that resolves once all internal resources have been released.

Example

import { Output, StreamTarget, Mp4OutputFormat } from '@mediabunny/browser';

const target = new StreamTarget();
const output = new Output({
  format: Mp4OutputFormat,
  target
});

// Add tracks
output.addVideoTrack(videoSource, {
  languageCode: 'eng',
  name: 'Main Video',
  frameRate: 30
});

output.addAudioTrack(audioSource, {
  languageCode: 'eng',
  name: 'Stereo Audio'
});

// Set metadata
output.setMetadataTags({
  title: 'My Video',
  author: 'John Doe'
});

// Start and process
await output.start();

// Add samples to sources...

await output.finalize();