Theprocess()method of anAudioWorkletProcessor-derived class implements the audioprocessing algorithm for the audio processor worklet.

Although the method isnot a part of theAudioWorkletProcessor interface, any implementationofAudioWorkletProcessor must provide aprocess() method.

The method is called synchronously from the audio rendering thread, once for each blockof audio (also known as a rendering quantum) being directed through the processor'scorrespondingAudioWorkletNode. In other words, every time a new block ofaudio is ready for your processor to manipulate, yourprocess() function isinvoked to do so.

Syntax

process(inputs, outputs, parameters)

Parameters

inputs

An array ofinputs connected to the node, each item of which is, in turn,an array ofchannels. Eachchannel is aFloat32Arraycontaining 128 samples. For example,inputs[n][m][i] will accessn-th input,m-th channel of that input, andi-th sampleof that channel.

Each sample value is in range of[-1 .. 1].

The number ofinputs and thus the length of that array is fixed at theconstruction of the node (seeAudioWorkletNode). If there isno active node connected to then-th input of the node,inputs[n] will be an empty array (zero input channels available).

The number ofchannels in each input may vary, depending onchannelCount andchannelCountMode properties.

outputs

An array ofoutputs that is similar to theinputs parameter instructure. It is intended to be filled during the execution of theprocess() method. Each of the output channels is filled with zeros bydefault — the processor will output silence unless the output arrays are modified.

parameters

An object containing string keys andFloat32Array values. For eachcustomAudioParam defined using theparameterDescriptorsgetter, the key in the object is aname of thatAudioParam, and the value is aFloat32Array. The valuesof the array are calculated by taking scheduled automation events intoconsideration.

If the automation rate of the parameter is"a-rate", the arraywill contain 128 values — one for each frame in the current audio block. If there'sno automation happening during the time represented by the current block, the arraymay contain a single value that is constant for the entire block, instead of 128identical values.

If the automation rate is"k-rate", the arraywill contain a single value, which is to be used for each of 128 frames.

Return value

A Boolean value indicating whether or not to force theAudioWorkletNodeto remain active even if theuser agent's internal logicwould otherwise decide that it's safe to shut down the node.

The returned value lets your processor have influence over the lifetime policy oftheAudioWorkletProcessor and the node that owns it. If the combinationof the return value and the state of the node causes the browser to decide to stop thenode,process() will not be called again.

Returningtrue forces the Web Audio API to keep the node alive,while returningfalse allows the browser to terminate the node if it isneither generating new audio data nor receiving data through its inputs that it isprocessing.

The 3 most common types of audio node are:

1. A source of output. AnAudioWorkletProcessor implementing such a nodeshould returntrue from theprocess method as long as itproduces an output. The method should returnfalse as soon as it's knownthat it will no longer produce an output. For example, take theAudioBufferSourceNode — the processor behind such a node should returntrue from theprocess method while the buffer is playing,and start returningfalse when the buffer playing has ended (there's noway to callplay on the sameAudioBufferSourceNode again).
2. A node that transforms its input. A processor implementing such a node should returnfalse from theprocess method to allow the presence ofactive input nodes and references to the node to determine whether it can begarbage-collected. An example of a node with this behavior is theGainNode. As soon as there are no inputs connected and referencesretained, gain can no longer be applied to anything, so it can be safelygarbage-collected.
3. A node that transforms its input, but has a so-calledtail-time — thismeans that it will produce an output for some time even after its inputs aredisconnected or are inactive (producing zero-channels). A processor implementing sucha node should returntrue from theprocess method for theperiod of thetail-time, beginning as soon as inputs are found that containzero-channels. An example of such a node is theDelayNode — it has atail-time equal to itsdelayTimeproperty.

Exceptions

As theprocess() method is implemented by the user, it can throw anything.If an uncaught error is thrown, the node will emit anprocessorerror event and willoutput silence for the rest of its lifetime.

Examples

In this example we create anAudioWorkletProcessor that outputs whitenoise to its first output. The gain can be controlled by thecustomGainparameter.

class WhiteNoiseProcessor extends AudioWorkletProcessor {  process(inputs, outputs, parameters) {    // take the first output    const output = outputs[0];    // fill each channel with random values multiplied by gain    output.forEach((channel) => {      for (let i = 0; i < channel.length; i++) {        // generate random value for each sample        // Math.random range is [0; 1); we need [-1; 1]        // this won't include exact 1 but is fine for now for simplicity        channel[i] =          (Math.random() * 2 - 1) *          // the array can contain 1 or 128 values          // depending on if the automation is present          // and if the automation rate is k-rate or a-rate          (parameters["customGain"].length > 1            ? parameters["customGain"][i]            : parameters["customGain"][0]);      }    });    // as this is a source node which generates its own output,    // we return true so it won't accidentally get garbage-collected    // if we don't have any references to it in the main thread    return true;  }  // define the customGain parameter used in process method  static get parameterDescriptors() {    return [      {        name: "customGain",        defaultValue: 1,        minValue: 0,        maxValue: 1,        automationRate: "a-rate",      },    ];  }}

Specifications

Browser compatibility

This is not a method provided by browsers, but a callback method that must be written in client code.

See also

• Using the Web Audio API