OfflineAudioContext

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since April 2021.

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.

EventTarget BaseAudioContext OfflineAudioContext

Constructor

OfflineAudioContext()

Creates a new OfflineAudioContext instance.

Instance properties

Also inherits properties from its parent interface, BaseAudioContext.

OfflineAudioContext.length Read only

An integer representing the size of the buffer in sample-frames.

Instance methods

Also inherits methods from its parent interface, BaseAudioContext.

OfflineAudioContext.suspend()

Schedules a suspension of the time progression in the audio context at the specified time and returns a promise.

OfflineAudioContext.startRendering()

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

OfflineAudioContext.resume()

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 AudioContext.resume) and thus can be accessed by both the AudioContext and OfflineAudioContext interfaces.

Events

Listen to these events using addEventListener() or by assigning an event listener to the oneventname property of this interface:

complete

Fired when the rendering of an offline audio context is complete.

Examples

Playing audio with an offline audio context

In this example, we declare both an AudioContext and an OfflineAudioContext object. We use the AudioContext to load an audio track fetch(), then the OfflineAudioContext to render the audio into an AudioBufferSourceNode and play the track through. After the offline audio graph is set up, we 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: You can run the full example live, or view the source.

js
// Define both online and offline audio contexts
let audioCtx; // Must be initialized after a user interaction
const offlineCtx = new OfflineAudioContext(2, 44100 * 40, 44100);

// Define constants for dom nodes
const play = document.querySelector("#play");

function getData() {
  // Fetch an audio track, decode it and stick it in a buffer.
  // Then we put the buffer into the source and can play it.
  fetch("viper.ogg")
    .then((response) => response.arrayBuffer())
    .then((downloadedBuffer) => audioCtx.decodeAudioData(downloadedBuffer))
    .then((decodedBuffer) => {
      console.log("File downloaded successfully.");
      const source = new AudioBufferSourceNode(offlineCtx, {
        buffer: decodedBuffer,
      });
      source.connect(offlineCtx.destination);
      return source.start();
    })
    .then(() => offlineCtx.startRendering())
    .then((renderedBuffer) => {
      console.log("Rendering completed successfully.");
      play.disabled = false;
      const song = new AudioBufferSourceNode(audioCtx, {
        buffer: renderedBuffer,
      });
      song.connect(audioCtx.destination);

      // Start the song
      song.start();
    })
    .catch((err) => {
      console.error(`Error encountered: ${err}`);
    });
}

// Activate the play button
play.onclick = () => {
  play.disabled = true;
  // We can initialize the context as the user clicked.
  audioCtx = new AudioContext();

  // Fetch the data and start the song
  getData();
};

Specifications

Specification
Web Audio API
# OfflineAudioContext

Browser compatibility

BCD tables only load in the browser

See also