jsdIScript.functionName semantics

The API for jsdIScript includes:

---
readonly char* functionName

Function name for this script. "anonymous" for unnamed functions (or a 
function actually named anonymous), empty for top level scripts. This 
data is copied from the underlying structure when the jsdIScript 
instance is created and is therefore available even after the script is 
invalidated.

(http://www.xulplanet.com/references/xpcomref/ifaces/jsdIScript.htm)
---

Experimentally (in Javascript) this value is false (null I guess) for
the script objects in stack frames created by eval().  I'm not sure what
"empty" means, but perhaps its means "in Javascript the value will test
false".  The eval() case is not mentioned in the doc. So would the be 
correct to say:

  empty for top level scripts and eval-created scripts.

And does that exhaust the possible causes of functionName == null?

In other words,  is "functionName == null plus a stack frame above the 
one containing this script" a test for an eval() frame?