Learning PHP, MySQL, JavaScript, CSS & HTML5, 3rd Edition

Chapter 15. Expressions and Control Flow in JavaScript

In the previous chapter, I introduced the basics of JavaScript and the DOM. Now it’s time to look at how to construct complex expressions in JavaScript and how to control the program flow of your scripts using conditional statements.

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 15-1 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 (remember that both <br> and <br /> are acceptable in HTML5, so I choose to use the former style for brevity).

Example 15-1. 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 both 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), actual strings of true and false are displayed.

In JavaScript, when you are checking whether a value is ...