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A JavaScript array is an object that contains a number of items. Those items can be variables (such as strings or numbers) or even other objects (such as Image objects or custom objects which you as a JavaScript developer have created). Once you have grouped all the items into the array you can then perfrom perform tasks like sorting the array items into alphabetical or numerical order, accessing and changing the value assigned to each array item , and passing the group of items as an argument to a JavaScript function (see [[Understanding JavaScript Functions]]) by passing just the array object.

A new instance of a JavaScript array object is created in the same way as any other object in JavaScript suing using the ''new'' keyword. For example we can create a new array object instance called ''myColors'' as follows:

'''Note:''' An array will grow automatically as new items are added so it is not necessary to specifiy at creation time the number of elements you believe the array will need. The decision as to whether to pre-allocate the initial size of the array is typically one of memory and speed. A mission critical application that needs to store a large amount of data in an array may not want the delay associated with waiting for the size of the array to be increased each time a new item is added. In this situation the array would be pre-initialized by specifying an initial size. This then has the trade-off that a large amount of memory may be taken up and not used right away. In practice it is usually acceptable to let array arrays grow as items are added.

before Before looking at how to add items to an array it is important to understand how arrays are indexindexed. the The first item in an array is item 0, the second is item 1 and so on.

Any time you need to change the value of an array elemt elementt the same approach applies. For example , to change the second elemt element of the array from "Green" to "Yellow":

muColorsmyColors[1] = "Yellow";

Any element of an array can be accessed by referencing the index into that array. For example , the following JavaScript will display the 3rd element of our ''myColors'' array (remembering that the first element is index 0) and display the string "Blue":

Often, however, it is necessary to access each element in an array. While writing code to access each item one by one is accepbtable for an array with one a few items , this becomes difficult for larger arrays. Fortunately thre are a couple of ways to ''loop'' through all the items in the array.

JavaScript provides an even easier way to access each element in an array using the ''for in'' syntax. This essentially performs the same functionas function as the above ''for loop'' but with a little less typing: