simple-filter-function.js

const getSourceNode = (audioContext, buffer) => {
  // let's create an empty buffer
  const source = audioContext.createBufferSource()

  // then we assign the passed buffer to node's buffer
  source.buffer = buffer
  // this is will play the buffer in looping
  source.looping = true

  return source
}

const getFilter = (audioContext, type, frequency) => {
  // let's create a filter node
  const filter = audioContext.createBiquadFilter()

  // then we assign the given type ('lowpass', 'highpass', etc)
  filter.type = type
  
  // let's set the frequency
  filter.frequency.value = frequency

  return filter
}

// now we need to create a source node with a buffer (like the previous examples)
const sourceNode = getSourceNode(audioContext, buffer)

// so we create a filter node
const filter = getFilter(audioContext, 'lowpass', 400)

// connecting the source node to the filter
sourceNode.connect(filter)

// and then filter to the speakers we'll be able hear the result
filter.connect(audioContext.destination)