Blame for src/mp3-worker.js

// Encodes PCM chunks to MP3 off the main thread, as they arrive.
import { Mp3Encoder } from '../vendor/lamejs/lamejs.js';

let enc = null, parts = [], job = 0;
onmessage = ({ data }) => {
  if (data.type === 'start') {
    job = data.job; parts = [];
    enc = new Mp3Encoder(1, data.sampleRate, data.kbps || 64);
  } else if (data.type === 'chunk' && data.job === job && enc) {
    const pcm = data.pcm, step = 1152 * 16;
    for (let i = 0; i < pcm.length; i += step) {
      const b = enc.encodeBuffer(pcm.subarray(i, i + step));
      if (b.length) parts.push(b);
    }
    postMessage({ type: 'encoded', job, index: data.index });
  } else if (data.type === 'finish' && data.job === job && enc) {
    const b = enc.flush();
    if (b.length) parts.push(b);
    postMessage({ type: 'done', job, blob: new Blob(parts, { type: 'audio/mpeg' }) });
    enc = null; parts = [];
  }
};