This function can be used to determine whether an expression holds an empty string. Because programmers tend to use the value undef where empty string would be more precise the function returns true if the argument is undef. Precisely:
The function returns true if the argument is undef or a string containing zero characters. Otherwise the function returns false.
See also ISARRAY, ISSTRING, ISINTEGER, ISREAL, ISNUMERIC, ISDEF, ISUNDEF, TYPE.