Data Types and Variables - Beginning JavaScript - page 43

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Now we're going to look at a new concept—something called an array . An array is similar to a
normal variable, in that you can use it to hold any type of data. However, it has one important
difference, which you see in this section.

As you have already seen, a normal variable can only hold one piece of data at a time. For example,
you can set myVariable to be equal to 25 like so:

myVariable = 25;

and then go and set it to something else, say 35 :

myVariable = 35;

However, when you set the variable to 35 , the first value of 25 is lost. The variable myVariable now
holds just the number 35 .
The following table illustrates the variable:

variaBle name value

myVariable 35

The difference between such a normal variable and an array is that an array can hold more than one
item of data at the same time. For example, you could use an array with the name myArray to store
both the numbers 25 and 35 . Each place where a piece of data can be stored in an array is called an
element .

How do you distinguish between these two pieces of data in an array? You give each piece of data
an index value. To refer to that piece of data, you enclose its index value in square brackets after
the name of the array. For example, an array called myArray containing the data 25 and 35 could be
illustrated using the following table:

elementname value

myArray[0] 25

myArray[1] 35

Notice that the index values start at 0 and not 1 . Why is this? Surely 1 makes more sense—after all,
we humans tend to say the first item of data, followed by the second item, and so on. Computers
start from 0 , and think of the first item as the zero item, the second as the first item, and so on.
Confusing, but you'll soon get used to this.

Arrays can be very useful because you can store as many (within the limits of the language, which
specifies a maximum of two to the power of 32 elements) or as few items of data in an array as
you want. Also, you don't have to say up front how many pieces of data you want to store in an
array.