javascript

Chapter 3: Language Basics
The following example illustrates this behavior:
alert(!false);
alert(!”blue”);
alert(!0);
alert(!NaN);
alert(!””);
alert(!12345);
//true
//false
//true
//true
//true
//false
The logical NOT operator can also be used to convert a value into its Boolean equivalent. By using two
NOT operators in a row, you can effectively simulate the behavior of the Boolean() casting function.
The first NOT returns a Boolean value no matter what operand it is given. The second NOT negates that
Boolean value and so gives the true Boolean value of a variable. The end result is the same as using the
Boolean() function on a value, as shown here:
alert(!!”blue”);
alert(!!0);
alert(!!NaN);
alert(!!””);
alert(!!12345);
//true
//false
//false
//false
//true
Logical AND
The logical AND operator is represented by the double ampersand ( & & ) and is applied to two values,
such as in this example:
var result = true & & false;
Logical AND behaves as described in the following truth table:
Operand 1 Operand 2 Result
True true true
True false false
False true false
False false false
Logical AND can be used with any type of operand, not just Boolean values. When either operand is not
a primitive Boolean, logical AND does not always return a Boolean value; instead, it does one of the
following:
❑
❑
❑
If the first operand is an object, then the second operand is always returned.
If the second operand is an object, then the object is returned only if the first operand evaluates
to true .
If both operands are objects, then the second operand is returned.
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