Javascript Arrays

From the MDN Web Docs

The Array object, as with arrays in other programming languages, enables storing a collection of multiple items under a single variable name, and has members for performing common array operations.

An array in JavaScript is a high-level, list-like object that is used to store multiple values in a single variable. Each value (also called an element) in an array has a numeric position, known as its index, and it can contain data of any type—numbers, strings, booleans, functions, objects, and even other arrays:

let fruits = ['apple', 'banana', 'cherry'];

Array Declaration

In JavaScript, an array can be declared in several ways:

  1. Array literal: This is the most common way to create an array. It uses square brackets [] and the elements are comma-separated.

    let fruits = ['apple', 'banana', 'cherry'];
  2. Array constructor: This uses the new keyword along with the Array constructor. It’s less common and more verbose.

    let fruits = new Array('apple', 'banana', 'cherry');
  3. Array.of() method: This creates a new Array instance from a variable number of arguments.

    let fruits = Array.of('apple', 'banana', 'cherry');
  4. Array.from() method: This creates a new Array instance from an array-like or iterable object.

    let fruits = Array.from(['apple', 'banana', 'cherry']);

In all these examples, fruits is an array that contains three strings. The strings are the elements of the array.

Array Indexes

In JavaScript, each item in an array is assigned a numeric position, known as its index. Array indices start at 0 and increment by 1 for each subsequent element.

let fruits = ['apple', 'banana', 'cherry'];

console.log(fruits[0]); // logs 'apple'
console.log(fruits[1]); // logs 'banana'
console.log(fruits[2]); // logs 'cherry'

Here, ‘apple’ is at index 0, ‘banana’ is at index 1, and ‘cherry’ is at index 2 in the fruits array.

You can use the index to access or modify the elements of the array:

fruits[1] = 'blueberry'; // changes 'banana' to 'blueberry'
console.log(fruits[1]); // logs 'blueberry'

In this case, fruits[1] = 'blueberry' changes the second element of the array to ‘blueberry’.

Array Length

The length property of an array in JavaScript returns the number of elements in the array. It’s not a method, so you don’t call it with parentheses.

let fruits = ['apple', 'banana', 'cherry'];
console.log(fruits.length); // logs 3

fruits.length is 3 because there are three elements in the fruits array.

You can also use the length property to change the number of elements in an array:

fruits.length = 2;
console.log(fruits); // logs ['apple', 'banana']

Setting fruits.length = 2 removes the last element from the array, so the array now only contains ‘apple’ and ‘banana’.

Array destructuring

Array destructuring in JavaScript is a syntax that allows you to unpack values from arrays, or properties from objects, into distinct variables.

let fruits = ['apple', 'banana', 'cherry'];

let [fruit1, fruit2, fruit3] = fruits;

console.log(fruit1); // logs 'apple'
console.log(fruit2); // logs 'banana'
console.log(fruit3); // logs 'cherry'

In this example, the array fruits is destructured into three new variables fruit1, fruit2, and fruit3. The first element of the array is assigned to fruit1, the second element to fruit2, and the third element to fruit3.

You can also skip over elements that you don’t need:

let [fruit1, , fruit3] = fruits;

console.log(fruit1); // logs 'apple'
console.log(fruit3); // logs 'cherry'

The second element of the array is skipped and not assigned to any variable.

The Spread Operator

The ... notation in JavaScript is known as the spread operator. It allows an iterable such as an array expression or string to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected.

Here’s an example of how to create a new array using the spread operator:

let fruits = ['apple', 'banana', 'cherry'];
let moreFruits = [...fruits, 'date', 'elderberry'];

console.log(moreFruits); // logs ['apple', 'banana', 'cherry', 'date', 'elderberry']

Here, ...fruits expands the elements of the fruits array into individual elements. The moreFruits array is a new array that contains all the elements of the fruits array, followed by ‘date’ and ‘elderberry’.

You can also use the spread operator to combine two arrays:

let fruits1 = ['apple', 'banana'];
let fruits2 = ['cherry', 'date'];
let allFruits = [...fruits1, ...fruits2];

console.log(allFruits); // logs ['apple', 'banana', 'cherry', 'date']

...fruits1 and ...fruits2 expand the elements of the fruits1 and fruits2 arrays into individual elements. The allFruits array is a new array that contains all the elements of the fruits1 and fruits2 arrays.