Learning PHP, MySQL & JavaScript, 7th Edition

Chapter 14. 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 by 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.

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.

Example 14-1. Four simple Boolean expressions

<script>
  console.log("a: " + (42 > 3))
  console.log("b: " + (91 < 4))
  console.log("c: " + (8 === 2))
  console.log("d: " + (4 < 17))
</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), the actual strings true and false are displayed.

In JavaScript, when you are checking whether a value is true or false, all values evaluate to true except the following, which evaluate to false:

• The string false itself
• 0
• –0
• The empty string
• null
• undefined
• NaN (Not a Number, a computer engineering concept for the result of an illegal floating-point operation such as division by zero).

Note that I am referring to