JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdn.jsdelivr.net/matreshka/latest/matreshka.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.32/browser.min.js"></script>
<script type="text/ecmascript-7">

class MyClass extends MK.Object {
	a = 1;

	constructor() {
  	super();

    this.on('change:a', function() {
      console.log('Париметр изменен и равен ' + this.a);
    });

    this.a = 5;
    this.a = 1;
	}
}

new MyClass();

</script>

JavaScript

Array.prototype.forEach.call(document.querySelectorAll('script[type="text/ecmascript-7"]'), function(script) {
	babel.run(script.innerHTML, {
		optional: ['es7.classProperties', 'es7.decorators', 'es7.functionBind']
	});
});