JavaScript Arrays

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

Displaying Arrays

In this tutorial we will use a script to display arrays inside a <p> element with id="demo":

Example

<p id="demo"></p>

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

The first line (in the script) creates an array named cars.

The second line "finds" the element with id="demo", and "displays" the array in the "innerHTML" of it. 

Try it Yourself

Create an array, and assign values to it:

Example

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

Try it Yourself »

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

Example

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

Try it Yourself »

Note Never put a comma after the last element (like "BMW",). The effect is inconsistent across browsers.

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:

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

Using the JavaScript Keyword new

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

Example

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

Try it Yourself »
Note 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 refer to an array element by referring to the index number.

This statement access the value of the first element in myCars:

var name = cars[0];

This statement modifies the first element in cars:

cars[0] = "Opel";
Note [0] is the first element in an array. [1] is the second. Array indexes start with 0.

You Can Have Different Objects in One Array

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;

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:

Array:

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

Try it Yourself »

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

Object:

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

Try it Yourself »

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 in cars
var y = cars.sort();         // The sort() method sort cars in alphabetical order

Array methods are covered in the next chapter.

The length Property

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

Example

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

Try it Yourself »

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

Adding Array Elements

The easiest way to add a new element to an array is to use the length property:

Example

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

Try it Yourself »

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

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[10] = "Lemon";                // adds a new element (Lemon) to fruits

Try it Yourself »

Looping Array Elements

The best way to loop through an array is using a standard for loop:

Example

var index;
var fruits = ["Banana", "Orange", "Apple", "Mango"];
for (index = 0; index < fruits.length; index++) {
    text += fruits[index];
}

Try it Yourself »

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 use numbered indexes.  

Example:

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"

Try it Yourself »

If you use a named index, when accessing an array, JavaScript will redefine the array to a standard object, and all array methods and properties will produce undefined or incorrect results.

 Example:

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

Try it Yourself »

The Difference Between Arrays and Objects?

In JavaScript, arrays use numbered indexes.  

In JavaScript, objects use named indexes.

Note 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

Try it Yourself »

The new keyword complicates your code and produces nasty side effects:

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 !!!!!

Try it Yourself »

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;             // typeof returns object

Try it Yourself »

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

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

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

Try it Yourself »

The function above always return true if the argument is an array.

Or more precisely: it returns true if the object prototype of the argument is "[object array]".


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