JavaScript Arrays

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

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

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

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

You can also create an array, and then provide the elements:

const cars = [];
cars[0]= "Saab";
cars[1]= "Volvo";
cars[2]= "BMW";

Using the JavaScript Keyword new

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

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

Accessing Array Elements

You access an array element by referring to the index number:

const cars = ["Saab", "Volvo", "BMW"];
let car = cars[0];

Changing an Array Element

This statement changes the value of the first element in cars:

cars[0] = "Opel";

const cars = ["Saab", "Volvo", "BMW"];
cars[0] = "Opel";

Converting an Array to a String

The JavaScript method toString() converts an array to a string of (comma separated) array values.

const fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.toString();

Access the Full Array

With JavaScript, the full array can be accessed by referring to the array name:

const cars = ["Saab", "Volvo", "BMW"];
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:

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

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

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

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).

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let length = fruits.length;

Accessing the First Array Element

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fruit = fruits[0];

Accessing the Last Array Element

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fruit = fruits[fruits.length - 1];

Looping Array Elements

One way to loop through an array, is using a for loop:

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>";

You can also use the Array.forEach() function:

const fruits = ["Banana", "Orange", "Apple", "Mango"];

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:

const fruits = ["Banana", "Orange", "Apple"];
fruits.push("Lemon");  // Adds a new element (Lemon) to fruits

New element can also be added to an array using the length property:

const fruits = ["Banana", "Orange", "Apple"];
fruits[fruits.length] = "Lemon";  // Adds "Lemon" to fruits

const fruits = ["Banana", "Orange", "Apple"];
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.  

const person = [];
person[0] = "John";
person[1] = "Doe";
person[2] = 46;
person.length;    // Will return 3
person[0];        // Will return "John"

const person = [];
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.

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:

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];

The new keyword can produce some unexpected results:

// Create an array with three elements:
const points = new Array(40, 100, 1);


// Create an array with two elements:
const points = new Array(40, 100);


// Create an array with one element ???
const points = new Array(40);  

const points = [40];

const points = new Array(40);

// Create an array with one element:
const points = [40];

// Create an array with 40 undefined elements:
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":

const fruits = ["Banana", "Orange", "Apple"];
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():

Array.isArray(fruits);

Solution 2:

The instanceof operator returns true if an object is created by a given constructor:

const fruits = ["Banana", "Orange", "Apple"];

(fruits instanceof Array);

Nested Arrays and Objects

Values in objects can be arrays, and values in arrays can be objects:

const myObj = {
  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:

for (let i in myObj.cars) {
  x += "<h1>" + myObj.cars[i].name + "</h1>";
  for (let j in myObj.cars[i].models) {
    x += myObj.cars[i].models[j];
  }
}