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ffmpeg.wasm is a pure Webassembly / Javascript port of FFmpeg. It enables video & audio record, convert and stream right inside browsers.

AVI to MP4 Demo


Try it:



$ npm install @ffmpeg/ffmpeg @ffmpeg/core

As we are using the latest experimental features, you need to add few flags to run in Node.js

$ node --experimental-wasm-threads --experimental-wasm-bulk-memory transcode.js


Or, using a script tag in the browser (only works in some browsers, see list below):

SharedArrayBuffer is only available to pages that are cross-origin isolated. So you need to host your own server with Cross-Origin-Embedder-Policy: require-corp and Cross-Origin-Opener-Policy: same-origin headers to use ffmpeg.wasm.

<script src="static/js/ffmpeg.min.js"></script>
  const { createFFmpeg } = FFmpeg;

Only browsers with SharedArrayBuffer support can use ffmpeg.wasm, you can check HERE for the complete list.


ffmpeg.wasm provides simple to use APIs, to transcode a video you only need few lines of code:

const fs = require('fs');
const { createFFmpeg, fetchFile } = require('@ffmpeg/ffmpeg');

const ffmpeg = createFFmpeg({ log: true });

(async () => {
  await ffmpeg.load();
  ffmpeg.FS('writeFile', 'test.avi', await fetchFile('./test.avi'));
  await'-i', 'test.avi', 'test.mp4');
  await fs.promises.writeFile('./test.mp4', ffmpeg.FS('readFile', 'test.mp4'));

Use other version of ffmpeg.wasm-core / @ffmpeg/core

For each version of ffmpeg.wasm, there is a default version of @ffmpeg/core (you can find it in devDependencies section of package.json), but sometimes you may need to use newer version of @ffmpeg/core to use the latest/experimental features.


Just install the specific version you need:

$ npm install @ffmpeg/core@latest

Or use your own version with customized path

const ffmpeg = createFFmpeg({
  corePath: '../../../src/ffmpeg-core.js',


const ffmpeg = createFFmpeg({
  corePath: 'static/js/ffmpeg-core.js',

For the list available versions and their changelog, please check:


Multi-threading need to be configured per external libraries, only following libraries supports it now:


Run it multi-threading mode by default, no need to pass any arguments.

libvpx / webm

Need to pass -row-mt 1, but can only use one thread to help, can speed up around 30%



What is the license of ffmpeg.wasm?

There are two components inside ffmpeg.wasm:

@ffmpeg/core contains WebAssembly code which is transpiled from original FFmpeg C code with minor modifications, but overall it still following the same licenses as FFmpeg and its external libraries (as each external libraries might have its own license).

@ffmpeg/ffmpeg contains kind of a wrapper to handle the complexity of loading core and calling low-level APIs. It is a small code base and under MIT license.

Can I use ffmpeg.wasm in Firefox?

Yes, but only for Firefox 79+ with proper header in both client and server, visit to try whether your Firefox works.

For more details: #106

What is the maximum size of input file?

2 GB, which is a hard limit in WebAssembly. Might become 4 GB in the future.

How can I build my own ffmpeg.wasm?

In fact, it is ffmpeg.wasm-core most people would like to build.

To build on your own, you can check inside repository.

Also you can check this series of posts to learn more fundamental concepts:

Why it doesn't work in my local environment?

When calling ffmpeg.load(), by default it looks for http://localhost:3000/node_modules/@ffmpeg/core/dist/ to download essential files (ffmpeg-core.js, ffmpeg-core.wasm, ffmpeg-core.worker.js). It is necessary to make sure you have those files served there.

If you have those files serving in other location, you can rewrite the default behavior when calling createFFmpeg():

const { createFFmpeg } = FFmpeg;
const ffmpeg = createFFmpeg({
  corePath: "http://localhost:3000/public/ffmpeg-core.js",
  // Use public address if you don't want to host your own.
  // corePath: ''
  log: true,