Document: doctype property
The doctype read-only property of the Document interface is a DocumentType object representing the the Document Type Declaration (DTD) associated with the current document.
Value
A DocumentType object.
Examples
js
const doctypeObj = document.doctype;
console.log(`doctypeObj.name: ${doctypeObj.name}`);
console.log(`doctypeObj.internalSubset: ${doctypeObj.internalSubset}`);
console.log(`doctypeObj.publicId: ${doctypeObj.publicId}`);
console.log(`doctypeObj.systemId: ${doctypeObj.systemId}`);
Notes
  The property returns null if there is no DTD associated with the current
  document.
DOM level 2 doesn't support editing the document type declaration.
Specifications
| Specification | 
|---|
| DOM Standard  # ref-for-dom-document-doctype①  | 
Browser compatibility
BCD tables only load in the browser