⚡ JavaScript Lesson 05 — Arrays

1. Array Basics

// Create
const fruits = ["apple", "banana", "mango"]
const numbers = [1, 2, 3, 4, 5]
const mixed = [1, "hello", true, null]  // mixed types OK

// Access by index (0-based)
fruits[0]   // "apple"
fruits[1]   // "banana"
fruits[2]   // "mango"

// Length
fruits.length  // 3

// Modify
fruits[0] = "pineapple"   // replace
fruits[3] = "orange"      // add at index 3

// Last element
fruits[fruits.length - 1]   // "orange"
fruits.at(-1)               // "orange" (modern)

2. Add & Remove Elements

let arr = [1, 2, 3]

// Add to end
arr.push(4)         // [1,2,3,4]
arr.push(5, 6)      // [1,2,3,4,5,6]

// Remove from end
arr.pop()           // removes 6, returns 6

// Add to front
arr.unshift(0)      // [0,1,2,3,4,5]

// Remove from front
arr.shift()         // removes 0, returns 0

// Remove/insert anywhere
arr.splice(2, 1)         // remove 1 element at index 2
arr.splice(2, 0, 99)     // insert 99 at index 2

3. map / filter / reduce — The Power Trio

const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

// map — transform each element → new array
const doubled = numbers.map(n => n * 2)
// [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

// filter — keep elements where condition=true → new array
const evens = numbers.filter(n => n % 2 === 0)
// [2, 4, 6, 8, 10]

// reduce — combine all into one value
const sum = numbers.reduce((acc, n) => acc + n, 0)
// 55

// Chaining!
const result = numbers
  .filter(n => n % 2 === 0)   // evens: [2,4,6,8,10]
  .map(n => n * n)            // squared: [4,16,36,64,100]
  .reduce((acc, n) => acc + n, 0)  // sum: 220

4. find / some / every / includes

const nums = [3, 7, 2, 9, 4, 1, 8]

// find — first match or undefined
nums.find(n => n > 5)      // 7

// findIndex — index of first match
nums.findIndex(n => n > 5) // 1

// some — at least one match? (boolean)
nums.some(n => n > 8)      // true (9 exists)

// every — all match? (boolean)
nums.every(n => n > 0)     // true

// includes — value exists? (boolean)
nums.includes(7)             // true
nums.includes(99)            // false

5. sort / reverse / flat / join

let arr = [3, 1, 4, 1, 5, 9, 2, 6]

// sort (alphabetically by default!)
arr.sort()              // [1, 1, 2, 3, 4, 5, 6, 9]

// sort numbers correctly
arr.sort((a, b) => a - b)   // ascending
arr.sort((a, b) => b - a)   // descending

// reverse
arr.reverse()

// join — array to string
["a", "b", "c"].join(", ")  // "a, b, c"
["a", "b", "c"].join("")    // "abc"

// flat — flatten nested array
[1, [2, 3], [4, [5]]].flat()      // [1,2,3,4,[5]]
[1, [2, 3], [4, [5]]].flat(2)     // [1,2,3,4,5]

// spread — copy array
const copy = [...arr]
const combined = [...arr1, ...arr2]

6. Quick Reference