JavaScript Arrays
An Array is an object type designed for storing data collections.
Key characteristics of JavaScript arrays are:
Elements: An array is a list of values, known as elements.
Ordered: Array elements are ordered based on their index.
Zero indexed: The first element is at index 0, the second at index 1, and so on.
Dynamic size: Arrays can grow or shrink as elements are added or removed.
Heterogeneous: Arrays can store elements of different data types (numbers, strings, objects and other arrays).
Why Use Arrays?
If you have a list of items (a list of car names, for example), storing the names in single variables could look like this:
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:
Note
It is a common practice to declare arrays with the const keyword.
Learn more about const with arrays in the chapter: JS Array Const.
Spaces and line breaks are not important. A declaration can span multiple lines:
You can also create an empty array, and provide elements later:
Using the JavaScript Keyword new
The following example also creates an Array, and assigns values to it:
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 array literal method.
Accessing Array Elements
You access an array element by referring to the index number:
let car = 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:
Converting an Array to a String
The JavaScript method toString() converts an array to a
string of (comma separated) array values.
Example
document.getElementById("demo").innerHTML = fruits.toString();
Result:
Access the Full Array
With JavaScript, the full array can be accessed by referring to the array name:
Example
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:
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[1] = myFunction;
myArray[2] = myCars;
Array Properties and Methods
The real strength of JavaScript arrays are the built-in array properties and methods:
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
let length = fruits.length;
The length property is always one more than the highest array index.
Accessing the First Array Element
Example
let fruit = fruits[0];
Accessing the Last Array Element
Example
let fruit = fruits[fruits.length - 1];
Looping Array Elements
One way to loop through an array, is using a for loop:
Example
let fLen = fruits.length;
let text = "<ul>";
for (let i = 0; i < fLen; i++) {
text += "<li>" + fruits[i] + "</li>";
}
text += "</ul>";
You can also use the Array.forEach() function:
Example
let 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
fruits.push("Lemon"); // Adds a new element (Lemon) to fruits
New element can also be added to an array using the length property:
Example
fruits[fruits.length] = "Lemon"; // Adds "Lemon" to fruits
WARNING !
Adding elements with high indexes can create undefined "holes" in an array:
Example
fruits[6] = "Lemon"; // Creates undefined "holes" in 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
person[0] = "John";
person[1] = "Doe";
person[2] = 46;
person.length; // Will return 3
person[0]; // Will return "John"
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:
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
person.length; // Will return 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.
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 = [];
These two different statements both create a new array containing 6 numbers:
const points = [40, 100, 1, 5, 25, 10];
The new keyword can produce some unexpected results:
const points = new Array(40, 100, 1);
const points = new Array(40, 100);
const points = new Array(40);
A Common Error
is not the same as:
const points = [40];
const points = new Array(40);
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":
let type = typeof fruits;
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():
Solution 2:
The instanceof operator returns true if an object is created
by a given constructor:
(fruits instanceof Array);
Nested Arrays and Objects
Values in objects can be arrays, and values in arrays can be objects:
Example
name: "John",
age: 30,
cars: [
{name:"Ford", models:["Fiesta", "Focus", "Mustang"]},
{name:"BMW", models:["320", "X3", "X5"]},
{name:"Fiat", models:["500", "Panda"]}
]
}
To access arrays inside arrays, use a for-in loop for each array:
Example
x += "<h1>" + myObj.cars[i].name + "</h1>";
for (let j in myObj.cars[i].models) {
x += myObj.cars[i].models[j];
}
}