OfflineAudioContext.startRendering (promise)

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The promise-based startRendering() method of the OfflineAudioContext Interface starts rendering the audio graph, taking into account the current connections and the current scheduled changes.

When the method is invoked, the rendering is started and a promise is raised. When the rendering is completed, the promise resolves with an AudioBuffer containing the rendered audio.


offlineAudioCtx.startRendering().then(function(renderedBuffer) {
  // do something with the output buffer

Return value

A Promise, which resolves with an AudioBuffer.


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();'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 audioCtx = new (window.AudioContext || window.webkitAudioContext)();
        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 'startRendering()' in that specification.
Working Draft Initial definition

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 42.0 37.0 (37.0) ? ? ?
Feature Chrome for Android Firefox Mobile (Gecko) Firefox OS IE Mobile Opera Mobile Safari Mobile Android Webview
Basic support 42.0 37.0 (37.0) 2.2 ? ? ? ?

See also

Document Tags and Contributors

 Contributors to this page: Sebastianz, Jeremie, fscholz, chRyNaN, chrisdavidmills
 Last updated by: Sebastianz,