JavaScript Arrays
JavaScript arrays are used to store multiple values in a single variable.
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:
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, ...];
Spaces and line breaks are not important. A declaration can span multiple lines:
You can also create an array, and then provide the elements:
Using the JavaScript Keyword new
The following example also creates an Array, and assigns values to it:
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:
const cars = ["Saab", "Volvo", "BMW"];
let x = cars[0]; // x = "Saab"
Try it Yourself »
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";
Access the Full Array
With JavaScript, the full array can be accessed by referring to the array name:
Example
const cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars;
Try it Yourself »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:
Objects use names to access its "members". In this example,
person.firstName
returns 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
const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.length; // Returns 4
Try it Yourself »
The length property is always one more than the highest array index.
Accessing the First Array Element
Example
const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[0]; // Returns "Banana"
Try it Yourself »
Accessing the Last Array Element
Example
const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[fruits.length - 1]; // Returns "Mango"
Try it Yourself »
Looping Array Elements
The safest way to loop through an array, is using a for loop:
Example
const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fLen = fruits.length;
text = "<ul>";
for (let i = 0; i < fLen; i++) {
text += "<li>" + fruits[i] + "</li>";
}
text
+= "</ul>";
Try it Yourself »
You can also use the Array.forEach() function:
Example
const fruits = ["Banana", "Orange", "Apple", "Mango"];
let text = "<ul>";
fruits.forEach(myFunction);
text += "</ul>";
function
myFunction(value) {
text += "<li>" + value + "</li>";
}
Try it Yourself »
Adding Array Elements
The easiest way to add a new element to an array is using the push() method:
Example
const fruits = ["Banana", "Orange", "Apple"];
fruits.push("Lemon"); // Adds a new element (Lemon) to fruits
Try it Yourself »
New element can also be added to an array using the length property:
Example
const fruits = ["Banana", "Orange", "Apple"];
fruits[fruits.length] = "Lemon"; // Adds "Lemon" to fruits
Try it Yourself »
WARNING !
Adding elements with high indexes can create undefined "holes" in an array:
Example
const fruits = ["Banana", "Orange", "Apple"];
fruits[6] = "Lemon"; // Creates undefined "holes" in fruits
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 always use numbered indexes.
Example
const person = [];
person[0] = "John";
person[1] = "Doe";
person[2] = 46;
person.length; // Will return 3
person[0]; // Will return "John"
Try it Yourself »
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:
const person = [];
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
person.length; // Will return 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.
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:
const points = new Array(); // Bad
const points = []; // Good
These two different statements both create a new array containing 6 numbers:
const points = new Array(40, 100, 1, 5, 25, 10);
const points = [40, 100, 1, 5, 25, 10];
Try it Yourself »
The new keyword only complicates the code. It can also
produce some unexpected results:
// This creates an array with two elements (40 and 100):
const points = new Array(40, 100);
What if I remove one of the elements?
// This creates an array with 40 undefined elements !!
const points = new Array(40);
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":
const fruits = ["Banana", "Orange", "Apple"];
typeof fruits; // returns object
Try it Yourself »
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
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Solution 2:
The instanceof operator returns true if an object is created
by a given constructor:
const fruits = ["Banana", "Orange", "Apple"];
fruits instanceof Array; // returns true
Try it Yourself »
