AudioWorkletProcessor: AudioWorkletProcessor() constructor

The AudioWorkletProcessor() constructor creates a new AudioWorkletProcessor object, which represents an underlying audio processing mechanism of an AudioWorkletNode.


Note: The AudioWorkletProcessor and classes that derive from it cannot be instantiated directly from a user-supplied code. Instead, they are created only internally by the creation of an associated AudioWorkletNodes.

new AudioWorkletProcessor(options)



An object that is passed as options parameter to the AudioWorkletNode constructor and passed through the structured clone algorithm. Available properties are as follows:

numberOfInputs Optional

The value to initialize the numberOfInputs property to. Defaults to 1.

numberOfOutputs Optional

The value to initialize the numberOfOutputs property to. Defaults to 1.

outputChannelCount Optional

An array defining the number of channels for each output. For example, outputChannelCount: [n, m] specifies the number of channels in the first output to be n and the second output to be m. The array length must match numberOfOutputs.

parameterData Optional

An object containing the initial values of custom AudioParam objects on this node (in its parameters property), with key being the name of a custom parameter and value being its initial value.

processorOptions Optional

Any additional data that can be used for custom initialization of the underlying AudioWorkletProcessor.

Note that there are default values for the first two properties, so even if there are no options object passed to the AudioWorkletNode constructor, the options object passed by the node to the AudioWorkletProcessor constructor will exist and at minimum have numberOfInputs and numberOfOutputs.

Return value

The newly constructed AudioWorkletProcessor instance.


In this example we pass custom options to the AudioWorkletNode constructor and observe how a structured clone of them gets passed to our AudioWorkletProcessor constructor.

First, we need to define a custom AudioWorkletProcessor and register it. Note that this should be done in a separate file.

// test-processor.js
class TestProcessor extends AudioWorkletProcessor {
  constructor(options) {
  process(inputs, outputs, parameters) {
    return true;

registerProcessor("test-processor", TestProcessor);

Next, in our main script file we'll load the processor, create an instance of AudioWorkletNode passing it the name of the processor and options object.

In the options object we pass processorOptions with a Map instance under someUsefulVariable key. We don't pass numberOfInputs and see how it gets its default value.

const audioContext = new AudioContext();
await audioContext.audioWorklet.addModule("test-processor.js");
const testNode = new AudioWorkletNode(audioContext, "test-processor", {
  processorOptions: {
    someUsefulVariable: new Map([
      [1, "one"],
      [2, "two"],

The console output will be as follows:

> 1 // AudioWorkletNode options.numberOfInputs set to default
> Map(2) { 1 => "one", 2 => "two" } // A cloned map under someUsefulVariable


Web Audio API
# dom-audioworkletprocessor-audioworkletprocessor

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See also