An array is a special variable, which can hold more than one value:
Why Use Arrays?
If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:let car1 = “Saab”;
let car2 = “Volvo”;
let 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:const array_name = [item1, item2, …];
It is a common practice to declare arrays with the const keyword.
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
const cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars;
</script>
</body>
</html>
Result:
JavaScript Arrays
Saab,Volvo,BMW
Spaces and line breaks are not important. A declaration can span multiple lines:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
const cars = [
"Saab",
"Volvo",
"BMW"
];
document.getElementById("demo").innerHTML = cars;
</script>
</body>
</html>
Result:
JavaScript Arrays
Saab,Volvo,BMW
You can also create an array, and then provide the elements:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
const cars = [];
cars[0]= "Saab";
cars[1]= "Volvo";
cars[2]= "BMW";
document.getElementById("demo").innerHTML = cars;
</script>
</body>
</html>
Result:
JavaScript Arrays
Saab,Volvo,BMW
Using the JavaScript Keyword new
The following example also creates an Array, and assigns values to it:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
const cars = new Array("Saab", "Volvo", "BMW");
document.getElementById("demo").innerHTML = cars;
</script>
</body>
</html>
Result:
JavaScript Arrays
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 array literal method.
Accessing Array Elements
You access an array element by referring to the index number:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>JavaScript array elements are accessed using numeric indexes (starting from 0).</p>
<p id="demo"></p>
<script>
const cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars[0];
</script>
</body>
</html>
Result:
JavaScript Arrays
JavaScript array elements are accessed using numeric indexes (starting from 0).
Saab
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>JavaScript array elements are accessed using numeric indexes (starting from 0).</p>
<p id="demo"></p>
<script>
const cars = ["Saab", "Volvo", "BMW"];
cars[0] = "Opel";
document.getElementById("demo").innerHTML = cars;
</script>
</body>
</html>
Result:
JavaScript Arrays
JavaScript array elements are accessed using numeric indexes (starting from 0).
Opel,Volvo,BMW
Access the Full Array
With JavaScript, the full array can be accessed by referring to the array name:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
const cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars;
</script>
</body>
</html>
Result:
JavaScript Arrays
Saab,Volvo,BMW
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:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Arrays use numbers to access its elements.</p>
<p id="demo"></p>
<script>
const person = ["John", "Doe", 46];
document.getElementById("demo").innerHTML = person[0];
</script>
</body>
</html>
Result:
JavaScript Arrays
Arrays use numbers to access its elements.
John
Objects use names to access its “members”. In this example, person.firstName returns John:
Object:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Objects</h2>
<p>JavaScript uses names to access object properties.</p>
<p id="demo"></p>
<script>
const person = {firstName:"John", lastName:"Doe", age:46};
document.getElementById("demo").innerHTML = person.firstName;
</script>
</body>
</html>
Result:
JavaScript Objects
JavaScript uses names to access object properties.
John
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:
cars.length // Returns the number of elements
cars.sort() // Sorts the array
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The length property returns the length of an array.</p>
<p id="demo"></p>
<script>
const fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.length;
</script>
</body>
</html>
Result:
JavaScript Arrays
The length property returns the length of an array.
4
The length property is always one more than the highest array index.
Accessing the First Array Element
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>JavaScript array elements are accesses using numeric indexes (starting from 0).</p>
<p id="demo"></p>
<script>
const fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits[0];
</script>
</body>
</html>
Result:
JavaScript Arrays
JavaScript array elements are accesses using numeric indexes (starting from 0).
Banana
Accessing the Last Array Element
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>JavaScript array elements are accesses using numeric indexes (starting from 0).</p>
<p id="demo"></p>
<script>
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits[fruits.length-1];
</script>
</body>
</html>
Result:
JavaScript Arrays
JavaScript array elements are accesses using numeric indexes (starting from 0).
Mango
Looping Array Elements
One way to loop through an array, is using a for loop:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The best way to loop through an array is using a standard for loop:</p>
<p id="demo"></p>
<script>
const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fLen = fruits.length;
let text = "<ul>";
for (let i = 0; i < fLen; i++) {
text += "<li>" + fruits[i] + "</li>";
}
text += "</ul>";
document.getElementById("demo").innerHTML = text;
</script>
</body>
</html>
Result:
JavaScript Arrays
The best way to loop through an array is using a standard for loop:
- Banana
- Orange
- Apple
- Mango
You can also use the Array.forEach() function:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Array.forEach() calls a function for each array element.</p>
<p id="demo"></p>
<script>
const fruits = ["Banana", "Orange", "Apple", "Mango"];
let text = "<ul>";
fruits.forEach(myFunction);
text += "</ul>";
document.getElementById("demo").innerHTML = text;
function myFunction(value) {
text += "<li>" + value + "</li>";
}
</script>
</body>
</html>
Result:
JavaScript Arrays
Array.forEach() calls a function for each array element.
- Banana
- Orange
- Apple
- Mango
Adding Array Elements
The easiest way to add a new element to an array is using the push() method:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The push method appends a new element to an array.</p>
<button onclick="myFunction()">Try it</button>
<p id="demo"></p>
<script>
const fruits = ["Banana", "Orange", "Apple"];
document.getElementById("demo").innerHTML = fruits;
function myFunction() {
fruits.push("Lemon");
document.getElementById("demo").innerHTML = fruits;
}
</script>
</body>
</html>
Result:
Result Shown in your Browser.
New element can also be added to an array using the length property:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The length property provides an easy way to append new elements to an array without using the push() method.</p>
<button onclick="myFunction()">Try it</button>
<p id="demo"></p>
<script>
const fruits = ["Banana", "Orange", "Apple"];
document.getElementById("demo").innerHTML = fruits;
function myFunction() {
fruits[fruits.length] = "Lemon";
document.getElementById("demo").innerHTML = fruits;
}
</script>
</body>
</html>
Result:
Result Shown in your Browser.
WARNING !
Adding elements with high indexes can create undefined “holes” in an array:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Adding elements with high indexes can create undefined "holes" in an array.</p>
<p id="demo"></p>
<script>
const fruits = ["Banana", "Orange", "Apple"];
fruits[6] = "Lemon";
let fLen = fruits.length;
let text = "";
for (i = 0; i < fLen; i++) {
text += fruits[i] + "<br>";
}
document.getElementById("demo").innerHTML = text;
</script>
</body>
</html>
Result:
JavaScript Arrays
Adding elements with high indexes can create undefined “holes” in an array.
Banana
Orange
Apple
undefined
undefined
undefined
Lemon
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
const person = [];
person[0] = "John";
person[1] = "Doe";
person[2] = 46;
document.getElementById("demo").innerHTML =
person[0] + " " + person.length;
</script>
</body>
</html>
Result:
JavaScript Arrays
John 3
WARNING !!
If you use named indexes, JavaScript will redefine the array to an object.
After that, some array methods and properties will produce incorrect results.
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>If you use a named index when accessing an array, JavaScript will redefine the array to a standard object, and some array methods and properties will produce undefined or incorrect results.</p>
<p id="demo"></p>
<script>
const person = [];
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
document.getElementById("demo").innerHTML =
person[0] + " " + person.length;
</script>
</body>
</html>
Result:
JavaScript Arrays
If you use a named index when accessing an array, JavaScript will redefine the array to a standard object, and some array methods and properties will produce undefined or incorrect results.
undefined 0
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.
JavaScript new Array()
JavaScript has a built in array constructor new Array().
But you can safely use [] instead.
These two different statements both create a new empty array named points:
const points = new Array();
const points = [];
These two different statements both create a new array containing 6 numbers:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Avoid using new Array(). Use [] instead.</p>
<p id="demo"></p>
<script>
//const points = new Array(40, 100, 1, 5, 25, 10);
const points = [40, 100, 1, 5, 25, 10];
document.getElementById("demo").innerHTML = points[0];
</script>
</body>
</html>
Result:
JavaScript Arrays
Avoid using new Array(). Use [] instead.
40
The new keyword can produce some unexpected results:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Create an Array with three elements.</p>
<p id="demo"></p>
<script>
var points = new Array(40, 100, 1);
document.getElementById("demo").innerHTML = points;
</script>
</body>
</html>
Result:
JavaScript Arrays
Create an Array with three elements.
40,100,1
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Create an Array with two elements.</p>
<p id="demo"></p>
<script>
var points = new Array(40, 100);
document.getElementById("demo").innerHTML = points;
</script>
</body>
</html>
Result:
JavaScript Arrays
Create an Array with two elements.
40,100
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Avoid using new Array().</p>
<p id="demo"></p>
<script>
var points = new Array(40);
document.getElementById("demo").innerHTML = points;
</script>
</body>
</html>
Result:
JavaScript Arrays
Avoid using new Array().
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
A Common Error
const points = [40];
is not the same as:const points = new Array(40);
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Create an Array with one element.</p>
<p id="demo"></p>
<script>
var points = [40];
document.getElementById("demo").innerHTML = points;
</script>
</body>
</html>
Result:
JavaScript Arrays
Create an Array with one element.
40
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Avoid using new Array().</p>
<p id="demo"></p>
<script>
var points = new Array(40);
document.getElementById("demo").innerHTML = points[0];
</script>
</body>
</html>
Result:
JavaScript Arrays
Avoid using new Array().
undefined
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“:
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The typeof operator, when used on an array, returns object:</p>
<p id="demo"></p>
<script>
const fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = typeof fruits;
</script>
</body>
</html>
Result:
JavaScript Arrays
The typeof operator, when used on an array, returns object:
object
The typeof operator returns object because a JavaScript array is an object.
Solution 1:
To solve this problem ECMAScript 5 (JavaScript 2009) defined a new method Array.isArray():
Example:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
const fruits = ["Banana", "Orange", "Apple"];
document.getElementById("demo").innerHTML = Array.isArray(fruits);
</script>
</body>
</html>
Result:
JavaScript Arrays
true
Solution 2:
The instanceof operator returns true if an object is created by a given constructor:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The instanceof operator returns true when used on an array:</p>
<p id="demo"></p>
<script>
var fruits = ["Banana", "Orange", "Apple"];
document.getElementById("demo").innerHTML = fruits instanceof Array;
</script>
</body>
</html>
Result:
JavaScript Arrays
The instanceof operator returns true when used on an array:
true