Microsoft® JScript® & Operator 
Language Reference Version 1

Performs a bitwise AND on two expressions.
result = expression1 & expression2The & operator syntax has these parts:
Part Description result Any variable. expression1 Any expression. expression2 Any expression.
The & operator looks at the binary representation of the values of two expressions and does a bitwise AND operation on them. The result of this operation behaves as follows:Any time both of the expressions have a 1 in a digit, the result has a 1 in that digit. Otherwise, the result has a 0 in that digit.0101 (expression1) 1100 (expression2)  0100 (result)For information on when a runtime error is generated by the & operator, see the Operator Behavior table.