Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

5.7 Logical Operators

The
logical operators are typically used to perform Boolean algebra. They
are often used in conjunction with comparison operators to express
complex comparisons that involve more than one variable and are
frequently used with the if,
while, and for statements.

5.7.1 Logical AND (&&)

When used with boolean
operands, the && operator performs the
Boolean AND operation on the two values: it returns
true if and only if both its first operand

and its second operand are
true. If one or both of these operands is
false, it returns false.

The actual behavior of this operator is somewhat more complicated. It
starts by evaluating its first operand, the expression on its left.
If the value of this expression can be converted to
false (for example, if the left operand evaluates
to null, 0,
", or undefined), the operator
returns the value of the lefthand expression. Otherwise, it evaluates
its second operand, the expression on its right, and returns the
value of that expression.[1]

5.7.2 Logical OR (||)

When used with boolean operands, the
|| operator performs the Boolean OR
operation on the two values: it returns true if
either the first operand

or the second operand
is true, or if both are true.
If both operands are false, it returns
false.

Although the || operator is most often used simply
as a Boolean OR operator, it, like the &&
operator, has more complex behavior. It starts by evaluating its
first operand, the expression on its left. If the value of this
expression can be converted to true, it returns
the value of the lefthand expression. Otherwise, it evaluates its
second operand, the expression on its right, and returns the value of
that expression.[2]

5.7.3 Logical NOT (!)

The ! operator is
a unary operator; it is placed before a single operand. Its purpose
is to invert the boolean value of its operand. For example, if the
variable a has the value true
(or is a value that converts to true),
!a has the value false. And if
the expression p &&
q evaluates to false (or to a
value that converts to false),
!(p &&
q) evaluates to true. Note that
you can convert any value x to a boolean value by
applying this operator twice: !!x.