JavaScript Arrays
An array is a special variable, which can hold more than one value:
const cars = ["Saab", "Volvo", "BMW"];
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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.
Learn more about const with arrays in the chapter: JS Array Const.
Example
const cars = ["Saab", "Volvo", "BMW"];
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Spaces and line breaks are not important. A declaration can span multiple lines:
Example
const cars = [
"Saab",
"Volvo",
"BMW"
];
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You can also create an array, and then provide the elements:
Example
const cars = [];
cars[0]= "Saab";
cars[1]= "Volvo";
cars[2]= "BMW";
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Using the JavaScript Keyword new
The following example also creates an Array, and assigns values to it:
Example
const cars = new Array("Saab", "Volvo", "BMW");
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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:
const cars = ["Saab", "Volvo", "BMW"];
let car = cars[0];
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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
const cars = ["Saab", "Volvo", "BMW"];
cars[0] = "Opel";
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Converting an Array to a String
The JavaScript method toString() converts an array to a
string of (comma separated) array values.
Example
const fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.toString();
Result:
Banana,Orange,Apple,Mango
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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;
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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:
const person = ["John", "Doe", 46];
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Objects use names to access its "members". In this example,
person.firstName
returns John:
Object:
const person = {firstName:"John", lastName:"Doe", age:46};
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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"];
let length = fruits.length;
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The length property is always one more than the highest array index.
Accessing the First Array Element
Example
const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fruit = fruits[0];
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Accessing the Last Array Element
Example
const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fruit = fruits[fruits.length - 1];
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Looping Array Elements
One way to loop through an array, is using a for loop:
Example
const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fLen = fruits.length;
let text = "<ul>";for (let i = 0; i < fLen; i++) { text += "<li>" + fruits + "</li>";}text
+= "</ul>";
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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>";}
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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
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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
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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
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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"
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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
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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:
const points = new Array(40, 100, 1, 5, 25, 10);
const points = [40, 100, 1, 5, 25, 10];
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The new keyword can produce some unexpected results:
// Create an array with three elements:
const points = new Array(40, 100, 1);
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// Create an array with two elements:
const points = new Array(40, 100);
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// Create an array with one element ???
const points = new Array(40);
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A Common Error
const points = [40];
is not the same as:
const points = new Array(40);
// Create an array with one element:
const points = [40];
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// Create an array with 40 undefined elements:
const points = new Array(40);
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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"];
let type = typeof fruits;
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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():
Array.isArray(fruits);
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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;
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Complete Array Reference
For a complete Array reference, go to our:
Complete JavaScript Array Reference.
The reference contains descriptions and examples of all Array
properties and methods.
Test Yourself With Exercises
Exercise:
Get the value "Volvo" from the cars array.
const cars = ["Saab", "Volvo", "BMW"];
let x = ;
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Start the Exercise
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Reference: https://www.w3schools.com/js/js_arrays.asp