String object versus literal - modifying the prototype?

Question

I\'m wondering why it seems that adding a method to the prototype of a string literal seems to work, but adding a property does not? I was playing with ideas in relation to

Answer 1

The string primitive is converted to a transient String object every time you try to invoke a method of the String object (the JavaScript engine internally converts a string primitive to a String object when necessary). After this function returns, the String object is (unobtrusively) converted back to a string primitive (under the hood) and this new primitive is returned (and most of the time assigned to a variable); every time a method of the String object is invoked.

So, after each invocation of testString._str_substr, _str_index1 is thrown away with the object and a new object (with a reset _str_index1) is created when _str_substr is called again.

See also MDC:

Because JavaScript automatically converts between string primitives and String objects, you can call any of the methods of the String object on a string primitive. JavaScript automatically converts the string primitive to a temporary String object, calls the method, then discards the temporary String object.