Java Reference
In-Depth Information
3
Decisions, Loops, and
Functions
So far, you've seen how to use JavaScript to get user input, perform calculations and tasks with
that input, and write the results to a web page. However, a pocket calculator can do all this, so
what is it that makes computers different? That is to say, what gives computers the appearance
of having intelligence? The answer is the capability to make decisions based on information
gathered.
How will decision-making help you in creating web sites? In the last chapter you wrote some
code that converted temperature in degrees Fahrenheit to centigrade. You obtained the degrees
Fahrenheit from the user using the prompt() function. This worked fi ne if the user entered a
valid number, such as 50. If, however, the user entered something invalid for the Fahrenheit
temperature, such as the string aaa, you would fi nd that your code no longer works as expected.
Now, if you had some decision-making capabilities in your program, you could check to see if
what the user has entered is valid. If it is, you can do the calculation, and if it isn't, you can tell
the user why and ask him to enter a valid number.
Validation of user input is probably one of the most common uses of decision making in
JavaScript, but it's far from being the only use.
In this chapter you'll look at how decision making is implemented in JavaScript and how you can
use it to make your code smarter.
Decision Making — The if and switch
Statements
All programming languages enable you to make decisions — that is, they enable the program to
follow a certain course of action depending on whether a particular
condition
is met. This is what
gives programming languages their intelligence.
