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4.3 JS Arrays

JavaScript arrays are used to store multiple values in a single variable.

Example: JS Arrays

var cars = ["Saab", "Volvo", "BMW"];

What is an Array?

An array is a special variable, which can hold more than one value at a time.
If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:

var car1 = "Saab";
var car2 = "Volvo";
var car3 = "BMW";

However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?
The solution is an array!
An array can hold many values under a single name, and you can access the values by referring to an index number.

Creating an Array

Using an array literal is the easiest way to create a JavaScript Array.
Syntax:

var array_name = [item1, item2, ...];

Example 1: JS Arrays part 1

var cars = ["Saab", "Volvo", "BMW"];

Spaces and line breaks are not important. A declaration can span multiple lines:

Example 2: JS Arrays part 2

var cars = [
  "Saab",
  "Volvo",
  "BMW"
];

Using the JavaScript Keyword new

The following example also creates an Array, and assigns values to it:

Example 3: JS Arrays part 3

var cars = new Array("Saab", "Volvo", "BMW");

The two examples above do exactly the same. There is no need to use new Array().
For simplicity, readability and execution speed, use the first one (the array literal method).

Access the Elements of an Array

You access an array element by referring to the index number.
This statement accesses the value of the first element in cars:

var name = cars[0];

Example 4: JS Arrays part 4

var cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars[0];

Note: Array indexes start with 0.
[0] is the first element. [1] is the second element.

Changing an Array Element

This statement changes the value of the first element in cars:

cars[0] = "Opel";

Example 5: JS Arrays part 5

var cars = ["Saab", "Volvo", "BMW"];
cars[0] = "Opel";
document.getElementById("demo").innerHTML = cars[0];

Access the Full Array

With JavaScript, the full array can be accessed by referring to the array name:

Example 6: JS Arrays part 6

var cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars;

Arrays are Objects

Arrays are a special type of objects. The typeof operator in JavaScript returns "object" for arrays.
But, JavaScript arrays are best described as arrays.
Arrays use numbers to access its "elements". In this example, person[0] returns John:

Example 7: JS Arrays part 7

var person = ["John", "Doe", 46];

Objects use names to access its "members". In this example, person.firstName returns John:

Object Example 8: JS Arrays part 8

var person = {firstName:"John", lastName:"Doe", age:46};

Array Elements Can Be Objects

JavaScript variables can be objects. Arrays are special kinds of objects.
Because of this, you can have variables of different types in the same Array.
You can have objects in an Array. You can have functions in an Array. You can have arrays in an Array

myArray[0] = Date.now;
myArray[1] = myFunction;
myArray[2] = myCars;

Array Properties and Methods

The real strength of JavaScript arrays are the built-in array properties and methods:

Examples

var x = cars.length;   // The length property returns the number of elements
var y = cars.sort();   // The sort() method sorts arrays

Array methods are covered in the next chapters.

The length Property

The length property of an array returns the length of an array (the number of array elements).

Example 9: JS Arrays part 9

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.length;   // the length of fruits is 4

The length property is always one more than the highest array index.

Accessing the First Array Element

Example 10: JS Arrays part 10

fruits = ["Banana", "Orange", "Apple", "Mango"];
var first = fruits[0];

Accessing the Last Array Element

Example 11: JS Arrays part 11

fruits = ["Banana", "Orange", "Apple", "Mango"];
var last = fruits[fruits.length - 1];

Looping Array Elements

Example 12: JS Arrays part 12

var fruits, text, fLen, i;
fruits = ["Banana", "Orange", "Apple", "Mango"];
fLen = fruits.length;

text = "<ul>";
for (i = 0; i < fLen; i++) {
  text += "<li>" + fruits[i] + "</li>";
}
text += "</ul>";

You can also use the Array.forEach() function:

Example 13: JS Arrays part 13

var fruits, text;
fruits = ["Banana", "Orange", "Apple", "Mango"];

text = "<ul>";
fruits.forEach(myFunction);
text += "</ul>";

function myFunction(value) {
  text += "<li>" + value + "</li>";
}

Adding Array Elements

The easiest way to add a new element to an array is using the push() method:

Example 14: JS Arrays part 14

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.push("Lemon");    // adds a new element (Lemon) to fruits

New element can also be added to an array using the length property:

Example 15: JS Arrays part 15

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[fruits.length] = "Lemon";    // adds a new element (Lemon) to fruits

WARNING!
Adding elements with high indexes can create undefined "holes" in an array:

Example 16: JS Arrays part 16

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[6] = "Lemon";    // adds a new element (Lemon) to fruits

Associative Arrays

Many programming languages support arrays with named indexes.
Arrays with named indexes are called associative arrays (or hashes).
JavaScript does not support arrays with named indexes.
In JavaScript, arrays always use numbered indexes.

Example 17: JS Arrays part 17

var person = [];
person[0] = "John";
person[1] = "Doe";
person[2] = 46;
var x = person.length;     // person.length will return 3
var y = person[0];         // person[0] will return "John"

WARNING!

If you use named indexes, JavaScript will redefine the array to a standard object.
After that, some array methods and properties will produce incorrect results.

Example 18: JS Arrays part 18

var person = [];
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
var x = person.length;     // person.length will return 0
var y = person[0];         // person[0] will return undefined

The Difference Between Arrays and Objects

In JavaScript, arrays use numbered indexes.
In JavaScript, objects use named indexes.

Arrays are a special kind of objects, with numbered indexes.

When to Use Arrays. When to use Objects.

JavaScript does not support associative arrays.
You should use objects when you want the element names to be strings (text).
You should use arrays when you want the element names to be numbers.

Avoid new Array()

There is no need to use the JavaScript's built-in array constructor new Array().
Use [] instead.
These two different statements both create a new empty array named points:

var points = new Array();     // Bad
var points = [];              // Good

These two different statements both create a new array containing 6 numbers:

var points = new Array(40, 100, 1, 5, 25, 10); // Bad
var points = [40, 100, 1, 5, 25, 10];          // Good

The new keyword only complicates the code. It can also produce some unexpected results:

var points = new Array(40, 100);  // Creates an array with two elements (40 and 100)

What if I remove one of the elements?

var points = new Array(40);  // Creates an array with 40 undefined elements !!!!!

How to Recognize an Array

A common question is: How do I know if a variable is an array?
The problem is that the JavaScript operator typeof returns "object":

var fruits = ["Banana", "Orange", "Apple", "Mango"];

typeof fruits;    // returns object

The typeof operator returns object because a JavaScript array is an object.

Solution 1:

To solve this problem ECMAScript 5 defines a new method Array.isArray():

Array.isArray(fruits);   // returns true

The problem with this solution is that ECMAScript 5 is not supported in older browsers.

Solution 2:

To solve this problem you can create your own isArray() function:

function isArray(x) {
  return x.constructor.toString().indexOf("Array") > -1;
}

The function above always returns true if the argument is an array.
Or more precisely: it returns true if the object prototype contains the word "Array".

Solution 3:

The instanceof operator returns true if an object is created by a given constructor:

var fruits = ["Banana", "Orange", "Apple", "Mango"];

fruits instanceof Array;   // returns true

JavaScript Fundamentals course home
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