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

3.3 Boolean Values

The number and string
data
types have a large or infinite number of possible values. The boolean
data type, on the other hand, has only two. The two legal boolean
values are represented by the literals true and
false. A boolean value represents a truth
value -- it says whether something is true or not.

Boolean values are generally the result of
comparisons you make in your
JavaScript programs. For example:

a == 4

This code tests to see if the value of the variable
a is equal to the number 4. If
it is, the result of this comparison is the boolean value
true. If a is not equal to
4, the result of the comparison is
false.

Boolean values are typically used in JavaScript control structures.
For example, the
if/else statement in JavaScript performs one
action if a boolean value is true and another
action if the value is false. You usually combine
a comparison that creates a boolean value directly with a statement
that uses it. The result looks like this:

if (a == 4)
b = b + 1;
else
a = a + 1;

This code checks if a equals 4.
If so, it adds 1 to b;
otherwise, it adds 1 to a.

Instead of thinking of the two possible boolean values as
true and false, it is sometimes
convenient to think of them as on
(true) and off
(false) or yes
(true) and no
(false). Sometimes it is even useful to consider
them equivalent to 1 (true) and
0 (false). (In fact, JavaScript
does just this and converts true and
false to 1 and
0 when necessary.)[3]

C programmers should
note that JavaScript has a distinct boolean data type, unlike C,
which simply uses integer values to simulate boolean values. Java
programmers should note that although JavaScript has a boolean type,
it is not nearly as pure as the Java boolean data
type -- JavaScript boolean values are easily converted to and from
other data types, and so in practice, the use of boolean values in
JavaScript is much more like their use in C than in Java.