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This is a continuation of a series on data structures in javascript. This week’s topic is the Array. An Array is an enumerated list of variables; meaning that each value can be referenced by a numerical index. The index number is referenced using square bracket syntax and Arrays in javascript are zero based.

document.writeln(someArray[0]);

Numerical based indexes make it easy to loop through an Array.

for (i=0; i < 10; i++) {
    document.writeln(someArray[i] + '<br>');
}

There are two different ways to create an Array in javascript. The first way is to use the new operator, but current best practices dictate that the new operator should not be used on javascript primitives.

var someArray = new Array(10);

However, since the new operator has a chance (very slim) to produce unexpected results, standard practice for creating an Array is to simply use square brackets.

var someArray = [];

If you have some experience with Arrays in other programming languages like C or C++, you may be wondering how to define the size of the Array. In javascript it is not necessary because the size will be automatically increased as needed.

Javascript Arrays can be initialized with data or you can create an empty Array and add data at a later time.

var someArray = [ 1, 2, 3 ];

var anotherArray = [];
anotherArray[0] = 1;
anotherArray[1] = 2;
anotherArray[2] = 3; 

You can skip indexes when populating the array and the value of the skipped indexes will be ‘undefined’

var someArray = [];
someArray[0] = 1;
someArray[1] = 2;
someArray[3] = 3;
document.writeln(someArray[2]);     // prints 'undefined' 

Arrays in other languages can be rigid about the type of data that is stored. Typically all of the data stored in the Array would need to be of the same type. This is not the case in javascript where you can store any type of data in the same Array. This means that you can add strings, functions, objects, booleans, or even additional Array’s to a single Array.

var someArray = [];
var someArray[0] = 1;
var someArray[1] = 'yojimbo';
var someArray[2] = {'firstName': 'John', 'lastName':'Doe'};
document.writeln(someArray[2].firstName); // prints 'John' 

An important characteristic of javascript Arrays to understand is that they are passed by reference to functions. This means that anything that you do inside of the function will alter the original Array.

var someArray = [0, 1,2];

function updateArray(passedArray) {
    passedArray[0] = 3;
} 

updateArray(someArray);
document.writeln(someArray[0]);    // prints '3'

Similarly, javascript Arrays are assigned by reference. This means that if you create an Array by assigning it to an existing Array, anything that you do with the second variable will alter the original Array.

var someArray = [0,1,2];
var anotherArray = someArray;

anotherArray[0] = 3;
document.writeln(someArray[0]);    // prints '3'

This is the first of a series of posts that will provide a beginner level introduction to data structures and algorithms in javascript.

A fundamental data structure in javascript is the Associative Array. An Associative Array is an abstract data type that is composed of a collection of key, value pairs such that each key exist only once in the collection.

The reason that an Associative Array is considered to be a fundamental data structure in javascript is that every object in the language is in fact an Associative Array.

Consider the following example:

var someArray = new Object();
someArray["A"] = 0;
someArray["B"] = 1;
someArray["C"] = 2;

You can get the values stored in the array by referencing the indexes or by object qualified naming:

alert(someArray["C"]);  // value by index
alert(someArray.C); // value by object qualified naming

An Associative Array can also be created using object literal notation:

var someArray = {A:0,  B:1, C:2};

Doing so creates the exact same array as the earlier example.

An important characteristic of the Associative Array in javascript is the fact that it is an object. In other languages, creation of an array requires that you explicitly define the data type. Any addition to the array that is of a different data type would result in an error. In javascript, the array object is defined by its members.

For example, you can add a method to an associative array:

var someArray = {A:0,  B:1, C:2}
someArray.Count = function() {
    return 3;
}

This is an important concept for beginners to understand because it may lead to some unexpected behavior. For example, looping through the array will show that there are four elements rather than three.

for(ind in someArray) {
    alert(ind);
}

This problem is typically solved in javascript by creating your own collection object.

var Collection = function() {
    this.count = 0;
    this.collection = {};
}

This defines a property for the count and an empty array. To make the collection object useful, it needs methods for adding and deleting elements from the collection array. These methods will update the count property.

this.add = function(key, item) {
    this.collection[key] = item;
    this.count  +=  1;
}
this.remove = function(key, item) {
    this.collection[key] = item;
    this.count  -=  1;
}

The collection object can now be used in the following manner:

var someCollection = new Collection();
someCollection.add("A", 0);
someCollection.add("B", 1);
someCollection.add("C", 2);
alert(someCollection.count);
alert(someCollection.collection["A"];
someCollection.remove("C");