The OfflineAudioContext interface is an AudioContext interface representing an audio-processing graph built from linked together AudioNodes. In contrast with a standard AudioContext, an OfflineAudioContext doesn't render the audio to the device hardware; instead, it generates it, as fast as it can, and outputs the result to an AudioBuffer.


Creates a new OfflineAudioContext instance.


Also inherits properties from its parent interface, BaseAudioContext.

OfflineAudioContext.length Read only
An integer representing the size of the buffer in sample-frames.

Event handlers

Is an EventHandler called when processing is terminated, that is when the complete event (of type OfflineAudioCompletionEvent) is raised, after the event-based version of OfflineAudioContext.startRendering() is used.


Also inherits methods from its parent interface, BaseAudioContext.

Schedules a suspension of the time progression in the audio context at the specified time and returns a promise.
Starts rendering the audio, taking into account the current connections and the current scheduled changes. This page covers both the event-based version and the promise-based version.

Deprecated methods

Resumes the progression of time in an audio context that has previously been suspended.

Note: The resume() method is still available — it is now defined on the BaseAudioContext interface (see BaseAudioContext.resume()) and thus can be accessed by both the AudioContext and OfflineAudioContext interfaces.


In this simple example, we declare both an AudioContext and an OfflineAudioContext object. We use the AudioContext to load an audio track via XHR (AudioContext.decodeAudioData), then the OfflineAudioContext to render the audio into an AudioBufferSourceNode and play the track through. After the offline audio graph is set up, you need to render it to an AudioBuffer using OfflineAudioContext.startRendering.

When the startRendering() promise resolves, rendering has completed and the output AudioBuffer is returned out of the promise.

At this point we create another audio context, create an AudioBufferSourceNode inside it, and set its buffer to be equal to the promise AudioBuffer. This is then played as part of a simple standard audio graph.

Note: For a working example, see our offline-audio-context-promise Github repo (see the source code too.)

// define online and offline audio context

var audioCtx = new AudioContext();
var offlineCtx = new OfflineAudioContext(2,44100*40,44100);

source = offlineCtx.createBufferSource();

// use XHR to load an audio track, and
// decodeAudioData to decode it and OfflineAudioContext to render it

function getData() {
  request = new XMLHttpRequest();

  request.open('GET', 'viper.ogg', true);

  request.responseType = 'arraybuffer';

  request.onload = function() {
    var audioData = request.response;

    audioCtx.decodeAudioData(audioData, function(buffer) {
      myBuffer = buffer;
      source.buffer = myBuffer;
      //source.loop = true;
      offlineCtx.startRendering().then(function(renderedBuffer) {
        console.log('Rendering completed successfully');
        var song = audioCtx.createBufferSource();
        song.buffer = renderedBuffer;


        play.onclick = function() {
      }).catch(function(err) {
          console.log('Rendering failed: ' + err);
          // Note: The promise should reject when startRendering is called a second time on an OfflineAudioContext


// Run getData to start the process off



Specification Status Comment
Web Audio API
The definition of 'OfflineAudioContext' in that specification.
Working Draft Initial definition

Browser compatibilityUpdate compatibility data on GitHub

Chrome Edge Firefox Internet Explorer Opera Safari
Basic support 14 Yes 25 No 15 6
OfflineAudioContext() constructor 55
Before Chrome 59, the default values were not supported.
? 53 No 42 ?
length 51 Yes Yes No 38 No
oncomplete 14 12 25 No 15 6
resume 49 Yes No No 36 No
suspend 49 Yes No No 36 No
startRendering 14 12 25 No 15 6
Android webview Chrome for Android Edge Mobile Firefox for Android Opera for Android iOS Safari Samsung Internet
Basic support Yes 18 Yes 26 15 ? Yes
OfflineAudioContext() constructor 55
Before version 59, the default values were not supported.
Before Chrome 59, the default values were not supported.
? 53 42 ? 6.0
length 51 51 Yes Yes 38 No 5.0
oncomplete Yes 18 Yes 26 15 ? Yes
resume 49 49 Yes No 36 No 5.0
suspend 49 49 Yes No 36 No 5.0
startRendering Yes 18 Yes 26 15 ? Yes

See also

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