Page 98 - DCAP202_Fundamentals of Web Programming

Unit 8: Programming Constructs in JavaScript

Notice also that the basic JavaScript syntax rules haven’t been broken – a semi-colon is present Notes
after both the true statement and the false statement.

Example:
if (80<100)
document.write(“The expression has evaluated to true!!”);
You can see the basic if statement used very simply here. Notice the “less-than” operator used
within the conditional expression - “80<100”. This if statement simply states that if 80 is less than
100, the statement is considered to be true. Since 80 is actually less than 100, the document.write
statement is written to the screen. If the conditional expression were “101<100” (which is read as
“if 101 is less than 100”), then nothing would be written to the screen as the conditional expression
would have evaluated to false. Use the if statement if you want something to happen only if the
conditional expression evaluates to true.

If you’d like an action to be taken when the conditional expression evaluates to false as well as
true, use the else addition to provide an action to be taken when the conditional expression
returns a false value, as shown below.
if (101<100) {
document.write(“The expression has evaluated to true!!”);
} else {
document.write(“The expression has evaluated to false!!”);
}
The above if/else statement says that if 101 is less than 100, write the first document.write statement
to the screen. If 101 isn’t less than 100 the second document.write statement is written to the screen.
Since 101 is not less than 100, the conditional expression returns false, and the second document.write
statement (“The expression has evaluated to false!!”) is written to the screen.

So now that you know the basic structure of the if and if/else statements, we’ll delve into
another aspect – nesting your if/else statements. This nesting structure is used when you have
an initial condition to be met with a true or false value, and then you require another set of
conditions that you’d like to have tested to acquire your end result.
Examine the below syntax example to understand more clearly what is meant by the term
“nesting”.
if (expression1) {
statement If expression1 is True;
} else {
if (expression2) {
statement If expression2 is True;
} else {
statement If expression2 is False;
}
}
You can see that the second if / else statement is nested within the “false” area of the first if / else
statement. If expression1 evaluates to true, then the first document.write statement of the first
if / else statement is written to the screen. If expression1 evaluates to false, then the second if /
else statement is evaluated. If expression2 evaluates to true, the first document.write statement
of the second if / else statement is written to the screen. If expression2 evaluates to false, then the
second document.write statement of the second if / else statement is written to the screen. This
somewhat complicated (but very useful) method will be used widely within your future JavaScript
coding.

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