javascript

Chapter 5: Reference Types
The Array Type
After the Object type, the Array type is probably the most used in ECMAScript. An ECMAScript array
is very different from arrays in most other programming languages. As in other languages, arrays are
ordered lists of data, but unlike in other languages, they can hold any type of data in each slot. This
means that it ’ s possible to create an array that has a string in the first position, a number in the second,
an object in the third, and so on. ECMAScript arrays are also dynamically sized, automatically growing
to accommodate any data that is added to them.
Arrays can be created in two basic ways. The first is to use the Array constructor, as in this line:
var colors = new Array();
If you know the number of items that will be in the array, you can pass the count into the constructor,
and the array will automatically be created with that number of slots (each of the items is initialized with
the value undefined ). For example, the following creates an array with 20 items:
var colors = new Array(20);
The Array constructor can also be passed items that should be included in the array. The following
creates an array with three string values:
var colors = new Array(“red”, “blue”, “green”);
An array can be created with a single value by passing it into the constructor. This gets a little bit tricky
because providing a single argument that is a number always creates an array with the given number of
items, whereas an argument of any other type creates a one - item array that contains the specified value.
Here ’ s an example:
var colors = new Array(3); //create an array with three items
var names = new Array(“Greg”); //create an array with one item, the string “Greg”
It ’ s possible to omit the new operator when using the Array constructor. It has the same result, as you
can see here:
var colors = Array(3); //create an array with three items
var names = Array(“Greg”); //create an array with one item, the string “Greg”
The second way to create an array is by using array literal notation. An array literal is specified by using
square brackets and placing a comma - separated list of items between them, as in this example:
var colors = [“red”, “blue”, “green”]; //creates an array with three strings
var names = [];
//creates an empty array
var values = [1,2,];
//AVOID! Creates an array with 2 or 3 items
var options = [,,,,,];
//AVOID! creates an array with 5 or 6 items
In this code, the first line creates an array with three string values. The second line creates an empty
array by using empty square brackets. The third line shows the effects of leaving a comma after the last
value in an array literal: In IE, values becomes a three - item array containing the values 1, 2, and
undefined ; in all other browsers, values is a two - item array containing the values 1 and 2.
This is due to a bug regarding array literals in the IE implementation of ECMAScript. Another instance
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