Arrays

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

Creating Arrays

const fruits = ["apple", "banana", "cherry"];
const numbers = [1, 2, 3, 4, 5];
const mixed = [42, "hello", true, null];
const empty = [];

Arrays use zero-based indexing:

console.log(fruits[0]);  // "apple"
console.log(fruits[2]);  // "cherry"
console.log(fruits[fruits.length - 1]);  // "cherry" (last element)

Adding and Removing Elements

const arr = ["a", "b", "c"];

arr.push("d");        // add to end => ["a", "b", "c", "d"]
arr.pop();            // remove from end => ["a", "b", "c"]
arr.unshift("z");     // add to start => ["z", "a", "b", "c"]
arr.shift();          // remove from start => ["a", "b", "c"]
arr.splice(1, 1);     // remove 1 element at index 1 => ["a", "c"]
arr.splice(1, 0, "x"); // insert "x" at index 1 => ["a", "x", "c"]

Searching Arrays

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

colors.indexOf("green");     // 1
colors.lastIndexOf("red");   // 3
colors.includes("blue");     // true
colors.includes("purple");   // false

// find — returns the first matching element
const nums = [5, 12, 8, 130, 44];
const found = nums.find(n => n > 10);  // 12

// findIndex — returns the index of the first match
const index = nums.findIndex(n => n > 10);  // 1

Transforming Arrays

const numbers = [1, 2, 3, 4, 5];

// map — create a new array by transforming each element
const squares = numbers.map(n => n ** 2);
// [1, 4, 9, 16, 25]

// filter — create a new array with only elements that pass the test
const evens = numbers.filter(n => n % 2 === 0);
// [2, 4]

// reduce — fold the array into a single value
const sum = numbers.reduce((acc, n) => acc + n, 0);
// 15

// sort — sorts in place (mutates the array)
const words = ["banana", "apple", "cherry"];
words.sort();  // ["apple", "banana", "cherry"]

// sort numbers (default sort is alphabetical!)
const nums2 = [10, 2, 30, 4];
nums2.sort((a, b) => a - b);  // [2, 4, 10, 30]

Combining and Slicing

const a = [1, 2];
const b = [3, 4];

const combined = a.concat(b);        // [1, 2, 3, 4]
const spread = [...a, ...b];         // [1, 2, 3, 4] (spread operator)