OfflineAudioContext: OfflineAudioContext() constructor

You can specify the parameters for the OfflineAudioContext() constructor as either the same set of parameters as are inputs into the BaseAudioContext.createBuffer method, or by passing those parameters in an options object. Either way, the individual parameters are the same.

numberOfChannels

An integer specifying the number of channels the resulting AudioBuffer should have.

length

An integer specifying the size of the buffer to create for the audio context, in sample-frames, where one sample-frame is a unit that can contain a single sample of audio data for every channel in the audio data. For example, a 5-second buffer with a sampleRate of 48000Hz would have a length of 5 * 48000 = 240000 sample-frames.

sampleRate

The sample-rate of the linear audio data in sample-frames per second. All user agents are required to support a range of 8000Hz to 96000Hz, and may support a wider range than that. The most commonly-used rate is 44100Hz, which is the sample rate used by CD audio.

It is important to note that, whereas you can create a new AudioContext using the AudioContext() constructor with no arguments, the OfflineAudioContext() constructor requires three arguments, since it needs to create an AudioBuffer. This works in exactly the same way as when you create a new AudioBuffer with the BaseAudioContext.createBuffer method. For more detail, read Audio buffers: frames, samples and channels from our Basic concepts guide.

A new OfflineAudioContext object whose associated AudioBuffer is configured as requested.

Like a regular AudioContext, an OfflineAudioContext can be the target of events, therefore it implements the EventTarget interface.

const offlineCtx = new OfflineAudioContext({
  numberOfChannels: 2,
  length: 44100 * 40,
  sampleRate: 44100,
});
const source = offlineCtx.createBufferSource();
// …

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