Operators - JavaScript by Example - page 105

EXPLANATION ( CONTINUED )

2 If the value of answer is any one of McDonald's or Taco Bell or Wendy's , the state-

ment enclosed within the curly braces is executed: An alert box appears display-
ing No fast food today, thanks . (See Figure 5.14.) If the user enters any other value,
nothing happens.

Figure 5.13 The user enters a value.

Figure 5.14 If the user enters any one of the values in line 2, this alert box appears.

The ! Operator (Logical NOT). In the English statement “That's not true!” the
word not is used for negation: Not true is false, and not false is true. JavaScript provides
the NOT ( ! ) operator for negation. The ! operator is called a unary operator because it
has only one operand; for example, ! true or ! 5 . It returns true if the expression evaluates
to false and returns false if the expression evaluates to true. See Table 5.14.

Table 5.14 Logical NOT Examples

Expression What It Evaluates To

! "this" false

! 0 true

! 2 false

! false true

! null true

! undefined true