Ebml parser and encoder

Usage no npm install needed!

<script type="module">
  import ebmlStream from 'https://cdn.skypack.dev/ebml-stream';


EBML stands for Extensible Binary Meta-Language and is somewhat of a binary version of XML. It's used for container formats like WebM or MKV.


This library was rewritten from version 3.0.0 of the existing npm ebml package. The rewrite was performed mainly to enable modification of tags during streaming. (At the time of this writing, the referenced library required a deep understanding of both the inner workings of the package and ebml to update the required properties correctly between decoding and encoding binary data). As part of the rewrite, the project was converted to use Typescript and to provide a uniform API for decoding and encoding EBML.


Install via NPM:

npm install --save ebml-stream


The EbmlStreamDecoder class is implemented as a Node Transform Stream. The input to this transform function should be binary EBML, provided in a Node Buffer object. The output of the stream is a series of EbmlTag objects.

These EbmlTag objects can then be modified as desired (encryption, compression, etc.) and reencoded using the EbmlStreamEncoder class. This class also extends Node Transform Stream. The input to this transform must be EbmlTag objects. The output of this transform function is binary EBML (in a Buffer) that can be written to disk or streamed to a client.

Note: When using EbmlStreamDecoder or EbmlStreamEncoder in a pipeline, make sure to set readableObjectMode=true and writableObjectMode=true for connected Transform streams.

EbmlTag Object

EbmlTag is an abstract class that specifies the basic data structure of an element in EBML. Creating new EBML tags can be done via the EbmlTagFactory.create method.

abstract class EbmlTag {
    // The id of the EBML tag.  In most documentation this number is in hexadecimal format
    id: number;
    // The data type of the EBML tag
    type: EbmlElementType;
    // The position of this EBML tag.  Currently, one of "Start", "Content", or "End"
    position: EbmlTagPosition;
    // The total size of the tag in bytes
    size: number;

    // Public abstract method that is overwritten in derived classes to encode tag content (everything after the "size" VInt)
    protected abstract encodeContent(): Buffer;
    // Public abstract method that is overwritten in derived classes to parse tag content from a raw Buffer
    public abstract parseContent(content: Buffer): void;
    // Public method that writes the current tag as binary EBML.  Depends on the `encodeContent` abstract method being correctly implemented by subclasses.
    public encode(): Buffer;

There are two base 'flavors' of EbmlTag:

  • EbmlMasterTag is a tag that contains one or more child tags. This tag always has a type of Master ('m'). When streaming, the EbmlStreamDecoder will first emit a master tag with position as "Start", then all child tags, then the master tag with position as "End".
  • EbmlDataTag is a tag that only contains data. This tag always has a position of "Content".

EbmlMasterTag Details

class EbmlMasterTag extends EbmlTag {
    Children: EbmlTag[];

This tag always has a type of Master ('m'). When streaming, this tag is only ever emitted with a position of "Start" or "End", and the tag's Children property will be empty (children of the tag will be emitted by the stream between the "Start" and "End" chunks). When encoding, if you wish to submit the tag by itself without individually pushing "Start", children, and "End" tags, you can set the tag's position to "Content". This will allow you to set the Children property in memory and write the tag once, rather than pushing each child separately.

EbmlDataTag Details

class EbmlDataTag extends EbmlTag {
    data: any;

This tag can contain data of any one of the defined Matroska data types:

  • UnsignedInt(u): Some of these are UIDs, coded as 128-bit numbers.
  • Integer(i): signed integer.
  • Float(f): IEEE-754 floating point number.
  • String(s): printable ASCII text string.
  • UTF8(8): printable utf-8 Unicode text string.
  • Date(d): a 64-bit signed timestamp, in nanoseconds after (or before) 2001-01-01T00:00UTC.
  • Binary(b): binary data, otherwise uninterpreted.

Regardless of the type of data stored, it can be retrieved from the data property of the class.

There are currently two known subtypes of EbmlDataTag that are treated as special cases:


class Block extends EbmlDataTag {
    payload: Buffer;
    track: number;
    value: number;
    invisible: boolean;
    lacing: BlockLacing;

These properties are specific to the Block element as defined by Matroska.


class SimpleBlock extends Block {
    discardable: boolean;
    keyframe: boolean; 

These properties are specific to the SimpleBlock element as defined by Matroska.


This example reads a media file into memory and decodes it.

const fs = require('fs');
const { EbmlStreamDecoder } = require('ebml-stream');

const decoder = new EbmlStreamDecoder();

decoder.on('data', chunk => console.log(chunk));

fs.readFile('media/test.webm', (err, data) => {
    if (err) {
        throw err;

This example does the same thing, but by piping the file stream into the decoder (a Transform stream). It also keeps track of the number of times each tag appears in the file.

const fs = require('fs');
const { EbmlStreamDecoder } = require('ebml-stream');

const ebmlDecoder = new EbmlStreamDecoder();
const tagCounts = {};

    .on('data', (tag) => {
        if (!tagCounts[tag.id]) {
            tagCounts[tag.id] = 0;
    .on('finish', () => console.log(tagCounts));

This example rips the audio from a webm and stores the result in a new file. The transform function in this example is rather advanced - an explanation follows the code.

const fs = require('fs');
const { EbmlStreamDecoder, EbmlStreamEncoder, EbmlTagId } = require('ebml-stream');
const { Transform } = require('stream');

const ebmlDecoder = new EbmlStreamDecoder({
    bufferTagIds: [
const ebmlEncoder = new EbmlStreamEncoder();

let strippedTracks = {};

    .pipe(new Transform({
        transform(chunk, enc, cb) {
            if(chunk.id === EbmlTagId.TrackEntry) {
                if(chunk.Children.find(c => c.id === EbmlTagId.TrackType).data != 2) {
                    strippedTracks[chunk.Children.find(c => c.id === EbmlTagId.TrackNumber).data] = true;
                    chunk = null;
            } else if(chunk.id === EbmlTagId.Block || chunk.id === EbmlTagId.SimpleBlock) {
                if(strippedTracks[chunk.track]) {
                    chunk = null;
            cb(null, chunk);
        readableObjectMode: true,
        writableObjectMode: true

In the above example, we (1) read a webm file from disk, (2) decode the webm file into an EbmlTag stream, (3) rip any tracks that are not audio out from the EbmlTag stream, (4) convert the EbmlTag stream back into binary, and (5) write the binary back to disk.

Steps 1, 2, 4, and 5 are rather straightforward (look into the other examples or read more about Node Stream Pipe functions for help), but step 3 (where we create the new Transform object) will likely require additional explanation.

Step 3 Breakdown

First, notice that we pass an additional option into the EbmlStreamDecoder constructor named bufferTagIds. This option tells the decoder which EbmlMasterTag objects should be fully parsed into "Content" tags before being emitted rather than the standard "Start" and "End" tags. This greatly simplifies our transform logic, as we don't have to maintain an internal buffer for the "TrackEntry" tag that we are interested in processing. Any tag ids that resolve to an EbmlDataTag will have no effect if they are supplied in this paramater. Second, we create the Transform object with the readableObjectMode and writableObjectMode properties set to true. Without those properties set, Node will throw an error because this stream works with objects rather than string or binary data.

Now, looking at the logic of the transform function itself -

  • First, we inspect the chunk to see if it is a "TrackEntry" tag. If so, we look through its Children to find the "TrackType" for this track. If the type is not 2 (audio), we add the track number to the strippedTracks object and set the chunk to null so that it is not passed through to the encoder.
  • If the chunk is not a "TrackEntry", we then check if it is a "Block" or a "SimpleBlock". If true, we check the trock number of the block. If the track is being stripped from the file, we set the chunk to null so that it will not be passed to the encoder.
  • The final line of the transform function merely passes the current chunk data through to the encoder so that it can be written to the output file.

State of this project

Parsing and writing should both work. If something is broken, please create an issue.

Any additional feature requests can also be submitted as an issue.

If any well-known tags have special parsing/encoding rules or data structures that aren't implemented, pull requests are welcome!




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