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

Conditionals

Conditionals alter program flow. They enable you to ask questions about certain things and respond to the answers you get in different ways. There are three types of non-looping conditionals: the if statement, the switch statement, and the ? operator.

The if Statement

Several examples in this chapter have already made use of if statements. The code within such a statement is executed only if the given expression evaluates to true. Multiline if statements require curly braces around them, but as in PHP, you can omit the braces for single statements. Therefore, the following statements are valid:

if (a > 100)
{
    b=2
    document.write("a is greater than 100")
}

if (b == 10) document.write("b is equal to 10")

The else statement

When a condition has not been met, you can execute an alternative using an else statement, like this:

if (a > 100)
{
    document.write("a is greater than 100")
}
else
{
    document.write("a is less than or equal to 100")
}

Unlike in PHP, there is no elseif statement, but that’s not a problem because you can use an else followed by another if to form the equivalent of an elseif statement, like this:

if (a > 100)
{
    document.write("a is greater than 100")
}
else if(a < 100)
{
    document.write("a is less than 100")
}
else
{
    document.write("a is equal to 100")
}

The else after the new if could equally be followed by another if statement, and so on. Also, although I have shown braces in these statements, because each is a single line the whole previous example could be written ...