The DocumentFragment.childElementCount read-only property returns the number of child elements of a DocumentFragment.
To get the number of children of a specific element, see Element.childElementCount.
A number representing the number of children of the document fragment.
let fragment = new DocumentFragment();
fragment.childElementCount; // 0
let paragraph = document.createElement("p");
fragment.appendChild(paragraph);
fragment.childElementCount; // 1