Node.isConnected
  The read-only isConnected property of the Node interface
  returns a boolean indicating whether the node is connected
  (directly or indirectly) to the context object,
  for example the Document object in the case of the normal DOM,
  or the ShadowRoot in the case of a shadow DOM.
Value
  A boolean value that is true if the node is connected to its relevant context object,
  and false if not.
Examples
Standard DOM
A standard DOM example:
let test = document.createElement('p');
console.log(test.isConnected); // Returns false
document.body.appendChild(test);
console.log(test.isConnected); // Returns true
Shadow DOM
A shadow DOM example:
// Create a shadow root
const shadow = this.attachShadow({mode: 'open'});
// Create some CSS to apply to the shadow dom
const style = document.createElement('style');
console.log(style.isConnected); // returns false
style.textContent = `
.wrapper {
  position: relative;
}
.info {
  font-size: 0.8rem;
  width: 200px;
  display: inline-block;
  border: 1px solid black;
  padding: 10px;
  background: white;
  border-radius: 10px;
  opacity: 0;
  transition: 0.6s all;
  positions: absolute;
  bottom: 20px;
  left: 10px;
  z-index: 3
}
`;
// Attach the created style element to the shadow dom
shadow.appendChild(style);
console.log(style.isConnected); // Returns true
Specifications
| Specification | 
|---|
| DOM Standard # ref-for-dom-node-isconnected① | 
Browser compatibility
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