Arrays are indexed by zero-based integers. In other words, to reference the first entry in an array named myArray, use myArray. A reference to the array entry returns the entry's value. To assign a value to an entry, use the simple assignment (=) operator. You may also use the add-by-value (+=) operator to add a number or append a string to an array entry, as appropriate.
One of the hazards of bringing too much object-oriented programming experience to scripting is that you might tend to turn every piece of data into an object, even when the overhead (in terms of source code space) to generate the object outweighs any functional or readability benefit you might get from using objects. It's not uncommon for an object-oriented approach to a simple problem to obfuscate the relationships among data points. But if your scripts frequently need to refer to some data associated with an entity that hangs around in the global variable space, it probably makes good sense to use an object there. In later chapters of this book, you will see many objects used as repositories for bits of information related to a particular item, such as the details of a drop-down menu.
3.0.3 Choosing Between Arrays and Objects
So, when do you use an array, and when do you use an object? Think of an array as an ordered list of similar kinds of data. The list, itself, signifies the purpose or kind of data it contains, such as a series of coworker names or the titles of all books on a shelf. The position of one item among the others is not important, although you might like to sort the list to, perhaps, show the contents in alphabetical order. Having the items in this kind of "blind" list means that at some point you will be looking through all items in the list, perhaps to pull out their values for insertion into an HTML element for display.
An object, on the other hand, is best used to encapsulate information about a single entity. A coworker object might contain properties for the person's name and age; a book object could have dozens of properties for information points such as title, author, publisher, category, and so on. The properties are explicitly named so that you can readily access the value of a single property directly (e.g., book3.publisher). You can also equip an object with methods whose actions operate on the object and its properties (see Recipe 3.8).
As you will see in Recipe 3.7 and elsewhere, there is an advantage in creating an array of objects. The "array-ness" gives your scripts the ability to iterate through the entire list of objects; the "object-ness" lets the same script inspect a specific property of each object in the array to perform tasks like lookups. An array of book objects, for instance, lets a looping construct look through the entire list of books and inspect the author property of each item to accumulate the title property values of those objects whose author property meets a particular criterion.
Be prepared to use arrays and objects by themselves, as well as in combination. Not only are you likely to use an array of objects, but a property of an object may be an array. For example, an object that represents a book might define the author property as an array to accommodate multiple authors. A book with a single author has a one-entry array for that property, but the scripts that go diving for authors know to expect an array data type there, as well as use appropriate comparison tools against the entries in the array.
As dry as this chapter's subject may seem at first glance, it may be the most important one in the entire book. Most of the recipes from this chapter are used in later chapters repeatedly because they are fundamental building blocks for a lot of Dynamic HTML and other scripting.
3.0.4 Getting Data into the Page
Most of the recipes in this chapter show data arrays and objects embedded directly within the page of the examples. This approach works for a fixed set of data or, after the page has loaded, data dynamically read from the page or user entry forms. But you can also embed data from live sources—server databases—with the help of server programming.