Arrays and Array Methods

Arrays are ordered collections of values. They are one of the most frequently used data structures in JavaScript, and the language provides a rich set of built-in methods for working with them.

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Creating Arrays

// Array literal (preferred)
const fruits = ["apple", "banana", "cherry"];

// Array constructor
const numbers = new Array(1, 2, 3);

// Empty array
const empty = [];

// Array.of() — creates an array from arguments
const single = Array.of(5); // [5] (not an empty array of length 5)

// Array.from() — creates an array from an iterable or array-like object
const letters = Array.from("hello"); // ["h", "e", "l", "l", "o"]

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Accessing and Modifying Elements

const colours = ["red", "green", "blue"];

// Access by index (zero-based)
console.log(colours[0]);   // "red"
console.log(colours[2]);   // "blue"

// Modify an element
colours[1] = "yellow";
console.log(colours);      // ["red", "yellow", "blue"]

// Array length
console.log(colours.length); // 3

// Access the last element
console.log(colours[colours.length - 1]); // "blue"
console.log(colours.at(-1));              // "blue" (ES2022)

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Adding and Removing Elements

Method	Action	Returns
push(item)	Add to the end	New length
pop()	Remove from the end	Removed element
unshift(item)	Add to the start	New length
shift()	Remove from the start	Removed element
splice(start, deleteCount, ...items)	Add/remove anywhere	Array of removed elements

const arr = [1, 2, 3];

arr.push(4);        // [1, 2, 3, 4]
arr.pop();          // [1, 2, 3]       returns 4
arr.unshift(0);     // [0, 1, 2, 3]
arr.shift();        // [1, 2, 3]       returns 0

// splice: remove 1 element at index 1, insert "a" and "b"
arr.splice(1, 1, "a", "b"); // [1, "a", "b", 3]   returns [2]

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Searching Arrays

Method	Description	Returns
indexOf(value)	First index of value, or -1	Number
lastIndexOf(value)	Last index of value, or -1	Number
includes(value)	Whether the array contains value	Boolean
find(fn)	First element matching the condition	Element or undefined
findIndex(fn)	Index of first match, or -1	Number

const nums = [10, 20, 30, 20, 40];

nums.indexOf(20);       // 1
nums.lastIndexOf(20);   // 3
nums.includes(30);      // true

nums.find(n => n > 25);      // 30
nums.findIndex(n => n > 25); // 2

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Iterating Over Arrays

forEach

Executes a function for each element (does not return a new array):

const names = ["Alice", "Bob", "Charlie"];
names.forEach((name, index) => {
  console.log(index + ": " + name);
});
// 0: Alice
// 1: Bob
// 2: Charlie

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Transforming Arrays

map

Creates a new array by transforming each element:

const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(n => n * 2);
console.log(doubled); // [2, 4, 6, 8, 10]

filter

Creates a new array with only elements that pass a test:

const numbers = [1, 2, 3, 4, 5, 6];
const evens = numbers.filter(n => n % 2 === 0);
console.log(evens); // [2, 4, 6]