Custom element with shadow not rendering

Question

I'm trying to make a custom element with a shadow, but when I add a shadow, the content of the element doesn't render. Here's my code:

JavaScript:

class CustomElement extends HTMLElement {
 constructor (){
  super();
  var shadow = this.attachShadow({mode: 'open'});
  var content = document.createElement("DIV");
  content.innerText = "hello world";
  shadow.appendChild(content);
 }
}
customElements.define("custom-element", CustomElement);

HTML:

<custom-element>blah blah blah</custom-element>

But all it renders is the text "hello world"

Answer 1

It's the normal behaviour of a Shadow DOM : the Shadow DOM content masks the original content (called the Light DOM).

If you want to reveal the Light DOM content, use <slot> in the Shadow DOM.

class CustomElement extends HTMLElement {
 constructor (){
  super();
  var shadow = this.attachShadow({mode: 'open'});
  var content = document.createElement("DIV");
  content.innerHTML = "hello world: <br> <slot></slot>";
  shadow.appendChild(content);
 }
}
customElements.define("custom-element", CustomElement);

<custom-element>blah blah blah</custom-element>