Undefined typically refers to something which has not yet been assigned a value (yet). Null refers to something which definitively has no value. In that case, I would recommend returning a null. Note that a function with no specified return value implicitly returns undefined.
From the ECMAScript2015 spec
4.3.10 undefined value
primitive value used when a variable has not been assigned a value
4.3.12 null value
primitive value that represents the
intentional absence of any object value
http://www.ecma-international.org/ecma-262/6.0/#sec-terms-and-definitions-undefined-type
Further reading: