Microsoft® JScript® && Operator 
Language Reference Version 1

Performs a logical conjunction on two expressions.
result = expression1 && expression2The && operator syntax has these parts:
Part Description result Any variable. expression1 Any expression. expression2 Any expression.
If, and only if, both expressions evaluate to True, result is True. If either expression evaluates to False, result is False.For information on when a runtime error is generated by the && operator, see the Operator Behavior table.
JScript uses the following rules for converting nonBoolean values to Boolean values:
 All objects are considered true.
 Strings are considered false if, and only if, they are empty.
 null and undefined are considered false.
 Numbers are false if, and only if, they are zero.