Learning PHP, MySQL, JavaScript, and CSS, 2nd Edition

Expressions

JavaScript expressions are very similar to those in PHP. As you learned in Chapter 4, an expression is a combination of values, variables, operators, and functions that results in a value; the result can be a number, a string, or a Boolean value (which evaluates to either true or false).

Example 14-1 shows some simple expressions. For each line, it prints out a letter between a and d, followed by a colon and the result of the expressions (the <br /> tag is there to create a line break and separate the output into four lines).

Example 14-1. Four simple Boolean expressions

<script>
document.write("a: " + (42 > 3) + "<br />")
document.write("b: " + (91 < 4) + "<br />")
document.write("c: " + (8 == 2) + "<br />")
document.write("d: " + (4 < 17) + "<br />")
</script>

The output from this code is as follows:

a: true
b: false
c: false
d: true

Notice that expressions a: and d: evaluate to true, but b: and c: evaluate to false. Unlike PHP (which would print the number 1 and nothing, respectively), JavaScript displays the actual strings “true” and “false”.

In JavaScript, when checking whether a value is true or false, all values evaluate to true with the exception of the following, which evaluate to false: the string false itself, 0, −0, the empty string, null, undefined, and NaN (Not a Number, a computer engineering concept for an illegal floating-point operation such as division by zero).

Note that I am referring to true and false in lowercase. This is because, unlike in PHP, these ...