Why can getter operations can implicitly return undefined?

[saschanaz]

There are two types of getter operations in the IDL world: returning null or undefined.

The first one is straightforward, it returns null when there is no item.

[Exposed=(Window,Worker)]
interface DOMStringList {
  readonly attribute unsigned long length;
  getter DOMString? item(unsigned long index);
  boolean contains(DOMString string);
};

But the second one is surprising. Even when the return type must be DOMString, it just returns undefined when there is no item.

[Exposed=Window,
 LegacyOverrideBuiltIns]
interface DOMStringMap {
  getter DOMString (DOMString name);
  [CEReactions] setter undefined (DOMString name, DOMString value);
  [CEReactions] deleter undefined (DOMString name);
};

Does the Web IDL spec allow this, or should this be specced? Or should it just be (DOMString or undefined) as it's now supported?

[annevk]

Isn't that because the property doesn't exist and therefore the getter is not invoked?

[saschanaz]

Ah thanks, you're right. Confirmed that all named getters either: 1) have nullable return types 2) returns an empty string 3) or just throws, with some exceptions in Web Speech. I'll file an issue there.