Working with Arrays, Loops, and Conditional Statements - HTML, CSS, and Dynamic HTML - page 833

Working with Array Length
A JavaScript array automatically expands in length as more items are added. If you need
to determine the array's current size, you use the property

array .length
where array is the name of the array. The value returned by the length property is
equal to one more than the highest index number in the array (because array indices start
at 0 rather than 1). This is not always the same as the number of array values. JavaScript
allows for the creation of sparse arrays , in which some array values are left undefined.
For example, the following commands create a sparse array in which only the first and
last items have defined values:

var x = new Array();
x[0] = “Maria”;
x[99] = “80517”;
The value of the length property for this array is 100 even though for the moment it only
contains two values. Sparse arrays occur frequently in database applications involving
customer records where items such as mobile phone numbers or postal codes have not
been entered for every person.
Note that you cannot reduce the value of the length property without removing items
from the end of your array. For example, the following command would reduce the
monthName array to the first three months—January, February, and March:

You can add new
items to the end of any
array using the com-
mand array [array.
length] = value;
where array is the name
of the array and value
is the new value to add to
the array.

monthName.length = 3;
There are also JavaScript methods for removing items from the front of an array, which
you'll explore later in this tutorial. Increasing the value of the length property adds
more items to an array, but the items have null values until they are defined.

Specifying Array Length
• To determine the size of an array, use the property

array .length
where array is the name of the array.
• To add an item to the end of an array, run the command

array [ i ] = value ;
where i is an index value higher than the highest index currently in the array. If you
don't know the highest index number, use the property array .length in place of i .
• To remove items from an array, run the command

array .length = value ;

where value is an integer that is smaller than the highest index currently in the array.

In the next section, you'll learn how to use JavaScript methods to reverse and
reorder arrays.