Javascript Audio Frequency Based Equalizer

equalizer.js

// Sample of setting
// const setting = {
//   left: [
//     [50, 0],
//     [1000, 0],
//     [2000, 0],
//     [4000, 0],
//     [8000, 0],
//     [12000, 0],
//     [15000, 0],
//     [18000, 1],
//   ],
//   right: [
//     [50, 1],
//     [1000, 0],
//     [2000, 0],
//     [4000, 0],
//     [8000, 0],
//     [12000, 0],
//     [15000, 0],
//     [18000, 0],
//   ],
// };

function bindSetting(context, splitter, merger, set, right = true) {
  const [
    frequency,
    gain,
  ] = set;
  const filter = context.createBiquadFilter();
  const gainNode = context.createGain();

  filter.type = 'bandpass';
  filter.Q.value = parseInt(filter.Q.maxValue, 10);
  filter.frequency.value = frequency;

  gainNode.gain.value = gain;

  splitter.connect(filter, 1);
  filter.connect(gainNode);
  gainNode.connect(merger, 0, right ? 1 : 0);
}

export default function loadAudio(url, setting = {}) {
  return new Promise((resolve, reject) => {
    if (!Array.isArray(setting.left) || !Array.isArray(setting.right)) {
      return reject(new Error('setting left and right should be array'));
    }

    const context = new AudioContext();

    return fetch(url)
      .then(response => response.arrayBuffer())
      .then(response => context.decodeAudioData(response, audioBuffer => {
        // Play Audio
        const source = context.createBufferSource();
        source.buffer = audioBuffer;

        const splitter = context.createChannelSplitter(2);
        const merger = context.createChannelMerger(2);

        source.connect(splitter);

        setting.left.forEach(set => bindSetting(context, splitter, merger, set, false));
        setting.right.forEach(set => bindSetting(context, splitter, merger, set, true));

        merger.connect(context.destination);

        resolve(source);
      }, e => reject(e))
    ).catch((ex) => reject(ex));
  });
}