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

This code declares a function named add that adds its arguments together and ―returns‖ the
resulting value. The return statement tells the interpreter what value the function evaluates to.
For example, you can set the value of the function equal to a variable:
var result = add(2, 3);
The arguments 2 and 3 are passed to the function, the body of the function executes, and the
result of their addition, 5, is placed in the variable result.
Besides passing in literal values to a function, it is also possible to pass in variables. For
example:
var a = 3, b=5;
var result;
result = add(a,b);
Experienced programmers might ask whether it is possible to modify the values of variables
that are passed in to functions. The answer is more a piece of advice: no. JavaScript employs
passing by value for primitive data types, so the values of the variables a and b should remain
unchanged regardless of what happens in the function add. However, other data types, notably
objects, can be changed when passed in (they are passed by reference), making the process
confusing to some. If you have programmed in other languages before, you will recognize that
functions are variously called procedures, subroutines, and methods. As you can see,
functions, which are discussed in detail in Chapter 6, are very powerful.
Input and Output in JavaScript
The ability to perform input and output (I/O) is an integral part of most languages. Because
JavaScript executes in a host environment like a Web browser, its I/O facilities might be
different from what you would expect. For obvious security reasons, plain client-side JavaScript
is not usually allowed to read or write files in the local file system. There are exceptions, but
these are considerably more advanced and will not be addressed until a later chapter.
I/O, like most useful tasks in JavaScript, is carried out through the objects provided by the
browser. Interacting with the user is typically achieved through the Window object, several
methods of which are described here. One of the most common I/O methods in JavaScript is
using the alert() method of Window, which displays its argument message in a dialog box that
includes an OK button. For example,
alert("This is an important message!");
causes the following dialog box to be presented to the user:
Other forms of dialog with the user include the confirm() method, which displays its argument
message in a dialog box with both OK and Cancel buttons. With the script
confirm("Learn JavaScript?");
you should see the following window: