The nextElementSibling read-only property returns the element immediately following the specified one in its parent's children list, or null if the specified element is the last one in the list.
The nextElementSibling read-only property returns the element immediately following the specified one in its parent's children list, or null if the specified element is the last one in the list.
MDN
The previousElementSibling read-only property returns the Element immediately prior to the specified one in its parent's children list, or null if the specified element is the first one in the list.
The previousElementSibling read-only property returns the Element immediately prior to the specified one in its parent's children list, or null if the specified element is the first one in the list.
MDN
https://dom.spec.whatwg.org/#nondocumenttypechildnode