JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

alter program flow. A variety of loops can be formed using while, for, or do-while in order to
iterate a particular piece of code. Further program flow control can be achieved with break and
continue. As larger scripts are built using the constructs presented in this chapter, repetitive
code is often introduced. To eliminate redundancy and create more modular programs,
functions—the topic of the next chapter—should be employed.
Chapter 5: Functions
JavaScript functions can be used to create script fragments that can be used over and over
again. When written properly, functions are abstract—they can be used in many situations and
are ideally completely self-contained, with data passing in and out through well-defined
interfaces. JavaScript allows for the creation of such functions, but many developers avoid
writing code in such a modular fashion and rely instead on global variables and side-effects to
accomplish their tasks. This is a shame, because JavaScript supports all the features
necessary to write modular code using functions and even supports some advanced features,
such as variable parameter lists. This chapter presents the basics of functions, and the next two
chapters discuss how, underneath it all, the real power of JavaScript comes from objects!
Function Basics
The most common way to define a function in JavaScript is by using the function keyword,
followed by a unique function name, a list of parameters (that might be empty), and a statement
block surrounded by curly braces. The basic syntax is shown here:
function functionname(parameter-list)
{
}
statements
A simple function that takes no parameters called sayHello is defined here:
function sayHello()
{
}
alert("Hello there");
To invoke the function somewhere later in the script, you would use the statement
sayHello();
Note Forward references to functions are generally not allowed; in other words, you should
always define a function before calling it. However, in the same tag within
which a function is defined you will be able to forward-reference a function. This is a very
poor practice and should be avoided.
Parameter-Passing Basics