OfflineAudioContext: startRendering() method

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

The complete event (of type OfflineAudioCompletionEvent) is raised when the rendering is finished, containing the resulting AudioBuffer in its renderedBuffer property.

Browsers currently support two versions of the startRendering() method — an older event-based version and a newer promise-based version. The former will eventually be removed, but currently both mechanisms are provided for legacy reasons.

Syntax

js
startRendering()

Parameters

None.

Return value

A Promise that fulfills with an AudioBuffer.

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
# dom-offlineaudiocontext-startrendering

Browser compatibility

BCD tables only load in the browser

See also