The number and string data types have an infinite number of possible values. The boolean data type, on the other hand, has only two. As we saw in Chapter 2, Lexical Structure, the two legal boolean values are the keywords true and false. A boolean value represents a "truth value"--it says whether something is true or not.
a == 4
if (a == 4) b = b + 1; else a = a + 1;