The DOMStringList interface is a legacy type returned by some APIs and represents a non-modifiable list of strings (DOMString).
DOMStringList
DOMString
MDN Reference
The read-only length property indicates the number of strings in the DOMStringList.
length
The contains() method returns a boolean indicating whether the given string is in the list.
contains()
The item() method returns a string from a DOMStringList by index.
item()