JavaScript Lesson 8: Arrays

Arrays in JavaScript work similarly to Python lists but with different method names. Arrays are everywhere in JS development.

Creating Arrays

const fruits = ["apple", "banana", "cherry"];
const numbers = [1, 2, 3, 4, 5];
const mixed = ["Alice", 30, true, null];
const empty = [];

console.log(fruits[0]);        // apple
console.log(fruits.length);    // 3
console.log(fruits[fruits.length - 1]);  // cherry (last)

Modifying Arrays

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

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

console.log(arr);

Searching Arrays

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

console.log(nums.indexOf(20));     // 1 (first occurrence)
console.log(nums.lastIndexOf(20)); // 3 (last occurrence)
console.log(nums.includes(30));    // true
console.log(nums.includes(99));    // false
console.log(nums.find(n => n > 25));     // 30
console.log(nums.findIndex(n => n > 25)); // 2

Copying and Combining

const a = [1, 2, 3];
const b = [4, 5, 6];

// Combine arrays
const combined = [...a, ...b];   // [1,2,3,4,5,6]
const concat = a.concat(b);      // same

// Copy array
const copy = [...a];             // [1,2,3] (not a reference!)
const copy2 = a.slice();         // same

// Slice (extract portion)
console.log(a.slice(1, 3));      // [2, 3]