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method

The

Array

object has a couple of methods that have equivalents in the

String

class, namely the

con-

cat()

and

slice()

methods. The

concat()

method works almost exactly the same as it does with

strings: The arguments are appended to the end of the array, and a new

Array

object (one containing

both the items in the original array and the new items) is returned as the function value. For example:

var aColors = [“red”, “green”, “blue”];

var aColors2 = arr.concat(“yellow”, “purple”);

alert(aColors2.toString()); //outputs “red,green,blue,yellow,purple”

alert(aColors.toString()); //outputs “red,green,blue”

In this example, the strings

“yellow”

and

“purple”

are added to the array using the

concat()

method. The

aColors2

array contains five values whereas the original array,

aColors

, still contains

only three. This can be proved by calling the

toString()

method on each array.

The

slice()

method is also very similar to

String

class equivalent in that it returns a new array con-

taining the specified items. Just like the

String

’s method, the

Array

’s

slice()

method can accept one

or two arguments: the starting and stopping positions of the items to extract. If only one argument is

present, the method returns all items between that position and the end of the array; if there are two

arguments, the method returns all items between the first position and second position, not including

the item in the second position. For example:

var aColors = [“red”, “green”, “blue”, “yellow”, “purple”];

var aColors2 = arr.slice(1);

var aColors3 = arr.slice(1, 4);

alert(aColors2.toString()); //outputs “green,blue,yellow,purple”

alert(aColors3.toString()); //outputs “green,blue,yellow”

Here,

aColors2

contains all the items in

arr

from position 1 on. Because the string

“green”

is in position

1, this is the first item in the new array. For

aColors3

, the

slice()

method is called with two arguments,

1 and 4. The string

“green”

is in position 1 and the string

“purple”

is in position 4, so

aColors3

con-

tains

“green”

“blue”

, and

“yellow”

because

slice()

only includes the item immediately before the

last position.

One of the interesting things about the ECMAScript

Array

class is that it provides methods to make an

array behave like other types of data structures. An

Array

object, for example, can act just like a stack,

which is one of a group of data structures that restrict the insertion and removal of items. A stack is

referred to as a

last-in-first-out

(LIFO) structure, meaning that the most recently added item is the first

one removed. The insertion and removal of items in a stack occur at only one point: the top of the stack.

It helps to think of a stack in literal terms, such as a stack of plates. If you want to add a plate to the stack

of plates, you place the plate on top of the stack. When an item is added to a stack data structure, it is

said to be

pushed

onto the stack; it is added at the top (Figure 3-1).

When it comes time to remove a plate for dinner, what do you do? You remove the top plate from the

stack of plates and put it on the table. Again, the stack data structure works the same way, removing

only the topmost item. When an item is removed from a stack, it is said to be

popped

from the stack

(Figure 3-2).

Object Basics

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