JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id=input placeholder="zzfxM pattern: [[[,1,,,...],[...]],[...]]"></textarea><br>
<button id=compress>Compress</button><br>
<textarea id=output placeholder="Self-compressing zzfxM pattern code"></textarea>

CSS

textarea { width: 100%; height: 15em; word-break: break-all; }

JavaScript

const jsonButEmpty = o => JSON.stringify(o).replace(/null/g, '');
const jsonButZero = o => JSON.stringify(o).replace(/null/g, '0');

compress.onclick = () => {
	const patterns = (0, eval)(input.value);
  const compressed = patterns.map(pattern => {
  	return pattern.map(([ins, pan, ...seq]) => {
    	return [ins, pan, seq.map(v => {
      	v = v || 0;
        const val = v | 0;
        const att = Math.round(v * 100) % 100;
        const att10 = att / 10 | 0;
        const att1 = att % 10;
        // note value: 37(-1) ,, 73(36)
        // attenuation (tens): 75(.1) 77(.2) .. 83(.9)
        // attenuation (ones): 74(.01) 76(.02) .. 82(.09)
        return String.fromCharCode(val + 38) +
        	(att10 ? String.fromCharCode(att10 * 2 + 73) : '') +
          (att1 ? String.fromCharCode(att1 * 2 + 72) : '');
      }).join('')];
		});
  });
  const code = jsonButEmpty(compressed) + `.map(p=>p.map(p=>[...p.pop()].map(b=>p.push((b=b.charCodeAt())<74?b-38:p.pop()+(b-72>>1)/(b&1?10:100)))&&p))`;
  const codeResult = jsonButZero((0, eval)(code));
  const origResult = jsonButZero(patterns);
	if (codeResult !== origResult) throw `verification failed:\n${origResult}\n${codeResult}`;
  output.value = code;
};