JSFiddle - React, Tailwind, and code Playground

HTML

<head>
		<title>test webcomponent</title>
		<script src="comp.js"></script> 
		<script>
			function myScript() {
				console.log("event!");
				alert ("myScript");
			}
		</script>
	</head>
	<body>
	
	
		<H1>test webcomponents</H1>
		<my-component id="xyz" bez="hallo" hello="myScript()"></my-component>
		
		<script>
			xyz.addEventListener("hello", function(event) {
				console.log(event.detail.name);
			});
		</script>
		
	</body>

JavaScript

class MyComponent extends HTMLElement {
	connectedCallback() {
		
		const shadow = this.attachShadow({mode: 'open'});
		
		var p=document.createElement("p");
		
		//in connectedCallback verwenden, nicht im konstruktor!
		var cap=this.getAttribute('bez');
		
		console.log("bez="+cap);
		
		var button=document.createElement("button");
		button.innerHTML=cap;
		button.addEventListener('click', () => {
			console.log("click");
			
			button.dispatchEvent(new CustomEvent("hello", {
				detail: { name: "John" }
			}));
		});
		
		shadow.appendChild(button);
	  }
	
/*
	
	attributeChangedCallback(name, oldValue, newValue) {
		if (oldValue === newValue) {
			return;
		}
		
		console.log(`The attribute ${name} has changed`);
	}
	*/
	
}

customElements.define('my-component',MyComponent);