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    js-synthesizer is a library that generates audio data (frames). WebAssembly (wasm) version of FluidSynth is used as a core synthesizer engine.



    npm install --save js-synthesizer


    From main thread

    Copies dist/js-synthesizer.js (or dist/js-synthesizer.min.js) and externals/libfluidsynth-2.2.1.js (libfluidsynth JS file) to your project, and writes <script> tags as following order:

    <script src="libfluidsynth-2.2.1.js"></script>
    <script src="js-synthesizer.js"></script>

    When scripts are available, please check whether waitForReady resolves.

    function loadSynthesizer() {
        // process with JSSynth...

    After initialized, you can use APIs via JSSynth namespace object.

    // Prepare the AudioContext instance
    var context = new AudioContext();
    var synth = new JSSynth.Synthesizer();
    // Create AudioNode (ScriptProcessorNode) to output audio data
    var node = synth.createAudioNode(context, 8192); // 8192 is the frame count of buffer
    // Load your SoundFont data (sfontBuffer: ArrayBuffer)
    synth.loadSFont(sfontBuffer).then(function () {
        // Load your SMF file data (smfBuffer: ArrayBuffer)
        return synth.addSMFDataToPlayer(smfBuffer);
    }).then(function () {
        // Play the loaded SMF data
        return synth.playPlayer();
    }).then(function () {
        // Wait for finishing playing
        return synth.waitForPlayerStopped();
    }).then(function () {
        // Wait for all voices stopped
        return synth.waitForVoicesStopped();
    }).then(function () {
        // Releases the synthesizer
    }, function (err) {
        console.log('Failed:', err);
        // Releases the synthesizer

    (Above example uses Web Audio API, but you can use Synthesizer without Web Audio, by using render() method.)

    js-synthesizer is built as UMD module. If you prefer to load js-synthesizer as a CommonJS / ES module, you can use import statement such as import * as JSSynth from 'js-synthesizer' by using bundlers (such as webpack).


    • js-synthesizer.js intends the ES2015-supported environment. If you need to run the script without errors on non-ES2015 environment such as IE11 (to notify 'unsupported'), you should load those scripts dynamically, or use transpiler such as babel.
    • When just after the scripts loaded, some APIs may fail since libfluidsynth is not ready. To avoid this, you can use the Promise object returned by JSSynth.waitForReady as above example.
    • libfluidsynth JS file is not import-able and its license (LGPL v2.1) is different from js-synthesizer's (BSD-3-Clause).

    With AudioWorklet

    js-synthesizer supports AudioWorklet process via dist/js-synthesizer.worklet.js (or dist/js-synthesizer.worklet.min.js). You can load js-synthesizer on the AudioWorklet as the following code:

    var context = new AudioContext();
        .then(function () {
            return context.audioWorklet.addModule('js-synthesizer.worklet.js');
        .then(function () {
            // Create the synthesizer instance for AudioWorkletNode
            var synth = new JSSynth.AudioWorkletNodeSynthesizer();
            // You must create AudioWorkletNode before using other methods
            // (This is because the message port is not available until the
            // AudioWorkletNode is created)
            audioNode = synth.createAudioNode(context);
            audioNode.connect(context.destination); // or another node...
            // After node creation, you can use Synthesizer methods
            return synth.loadSFont(sfontBuffer).then(function () {
                return synth.addSMFDataToPlayer(smfBuffer);
            }).then(function () {
                return synth.playPlayer();
            }).then(function () {

    With Web Worker

    js-synthesizer and libfluidsynth can be executed on a Web Worker. Executing on a Web Worker prevents from blocking main thread while rendering.

    To use js-synthesizer on a Web Worker, simply call importScripts as followings:


    Note that since the Web Audio is not supported on the Web Worker, the APIs/methods related to the Web Audio will not work. If you want to use both Web Worker and AudioWorklet, you should implement AudioWorkletProcessor manually as followings:

    • main thread -- create AudioWorkletNode and establish connections between Web Worker and AudioWorklet
      • You need to transfer rendered audio frames from Web Worker to AudioWorklet, but AudioWorklet environment does not support creating Web Worker. By creating MessageChannel and sending its port instances to Web Worker and AudioWorklet, they can communicate each other directly.
    • Web Worker thread -- render audio frames into raw buffers and send it for AudioWorklet thread
    • AudioWorklet thread -- receive audio frames (and queue) and 'render' it in the process method


    Creation of Synthesizer instance

    These classes implement the interface named JSSynth.ISynthesizer.

    • JSSynth.Synthesizer (construct: new JSSynth.Synthesizer())
      • Creates the general synthesizer instance. No parameters are available.
    • JSSynth.AudioWorkletNodeSynthesizer (construct: new JSSynth.AudioWorkletNodeSynthesizer())
      • Creates the synthesizer instance communicating AudioWorklet (see above). No parameters are available.
      • You must call createAudioNode method first to use other instance methods.

    Creation of Sequencer instance

    The Sequencer instance is created only via following methods:

    • JSSynth.Synthesizer.createSequencer (static method)
      • Returns the Promise object that resolves with JSSynth.ISequencer instance. The instance can be used with JSSynth.Synthesizer instances.
    • JSSynth.AudioWorkletNodeSynthesizer.prototype.createSequencer (instance method)
      • Returns the Promise object that resolves with JSSynth.ISequencer instance. The instance can be used with JSSynth.AudioWorkletNodeSynthesizer instances which handled createSequencer calls.

    Using hook / handle MIDI-related event data with user-defined calllback

    You can hook MIDI events posted by player. For JSSynth.Synthesizer instance, use hookPlayerMIDIEvents method as followings:

    syn.hookPlayerMIDIEvents(function (s, type, event) {
        // hook '0xC0' event (Program Change event)
        if (type === 0xC0) {
            // if the 'program' value is 0, use another SoundFont
            if (event.getProgram() === 0) {
                syn.midiProgramSelect(event.getChannel(), secondSFont, 0, 0);
                return true;
        // return false to use default processings for other events
        return false;

    For JSSynth.AudioWorkletNodeSynthesizer instance, use hookPlayerMIDIEventsByName as followings:

    • worklet.js
    // We must add method to AudioWorkletGlobalScope to pass to another module.
    AudioWorkletGlobalScope.myHookPlayerEvents = function (s, type, event, data) {
        if (type === 0xC0) {
            if (event.getProgram() === 0) {
                // 'secondSFont' will be passed from 'hookPlayerMIDIEventsByName'
                s.midiProgramSelect(event.getChannel(), data.secondSFont, 0, 0);
                return true;
        return false;
    • main.js
    // before use this, 'worklet.js' above must be loaded as AudioWorklet completely, and
    // syn.createAudioNode must be called to activate worklet.
    // The first parameter is the method name added to 'AudioWorkletGlobalScope'.
    // The second parameter will be passed to the worklet.
    syn.hookPlayerMIDIEventsByName('myHookPlayerEvents', { secondSFont: secondSFont });

    The sequencer also supports 'user-defined client' to handle event data.

    • For sequncer instance created by Synthesizer.createSequencer, use Synthesizer.registerSequencerClient static method.
      • You can use Synthesizer.sendEventNow static method to send event data, processed by the synthesizer or clients, to another clients/synthesizers.
    • For sequncer instance created by createSequencer of AudioWorkletNodeSynthesizer, use registerSequencerClientByName instance method.
      • The callback function must be added to 'AudioWorkletGlobalScope' like hookPlayerMIDIEventsByName's callback.
      • To re-send event data, use Synthesizer.sendEventNow in the worklet. Synthesizer constructor is available via AudioWorkletGlobalScope.JSSynth.Synthesizer.
    • You can rewrite event data passed to the callback, by using JSSynth.rewriteEventData (AudioWorkletGlobalScope.JSSynth.rewriteEventData for worklet).

    JSSynth methods


    Can be used to wait for the synthesizer engine's ready.

    Return: Promise object (resolves when the synthesizer engine (libfluidsynth) is ready)

    JSSynth.ISynthesizer methods

    (Not documented yet. Please see dist/lib/ISynthesizer.d.ts.)


    js-synthesizer is licensed under BSD 3-Clause License except for the files in externals directory. For licenses of the files in externals directory, please read externals/


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