Testcase

var a = '123'.substr(1, undefined);
print(a);

Expect Output

'23'

Rhino Actual Output

''

Other JS Engines Output

'23'

Rhino Version

1.7.12 & 1.7.11

ECMA Script Standard

In ES Standard( 5.1/6/10), it says:

The substr method takes two arguments, start and length, and returns a substring of the result of converting the this object to a String, starting from character position start and running for length characters (or through the end of the String if length is undefined).

I think this is a implementation bug of rhino.

Looking forward to your reply, thanks! 😆