Arrays & Array Methods

// Array literal (preferred)
const fruits = ["apple", "banana", "cherry"];
const numbers = [1, 2, 3, 4, 5];
const mixed = ["hello", 42, true, null, { name: "Alice" }];

// Array constructor (avoid)
const arr = new Array(3);  // Creates array with 3 empty slots

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

// Access by index (0-based)
console.log(colors[0]);    // => "red"
console.log(colors[2]);    // => "blue"
console.log(colors[-1]);   // => undefined (no negative indexing like Python)

// Length
console.log(colors.length); // => 3

// Modify
colors[1] = "yellow";
console.log(colors);        // => ["red", "yellow", "blue"]

// Last element
console.log(colors[colors.length - 1]);  // => "blue"

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

// End
items.push("d");          // Add to end
console.log(items);        // => ["a", "b", "c", "d"]
const last = items.pop(); // Remove from end
console.log(last);         // => "d"
console.log(items);        // => ["a", "b", "c"]

// Beginning
items.unshift("z");        // Add to beginning
console.log(items);        // => ["z", "a", "b", "c"]
const first = items.shift(); // Remove from beginning
console.log(first);        // => "z"
console.log(items);        // => ["a", "b", "c"]

const numbers = [10, 20, 30, 40, 50];

// indexOf — returns index or -1
console.log(numbers.indexOf(30));    // => 2
console.log(numbers.indexOf(99));    // => -1

// includes — boolean
console.log(numbers.includes(30));   // => true
console.log(numbers.includes(99));   // => false

// find — returns first matching element
const found = numbers.find(n => n > 25);
console.log(found);                  // => 30

// findIndex — returns index of first match
const index = numbers.findIndex(n => n > 25);
console.log(index);                  // => 2

// some/every — boolean checks
console.log(numbers.some(n => n > 40));   // => true (at least one)
console.log(numbers.every(n => n > 5));   // => true (all)

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

// slice — returns a new array (original unchanged)
console.log(arr.slice(1, 3));     // => ["b", "c"]  (start at 1, end before 3)
console.log(arr.slice(2));         // => ["c", "d", "e"]  (from 2 to end)
console.log(arr.slice(-2));        // => ["d", "e"]  (last 2)

// splice — modifies original array (add/remove at index)
const arr2 = ["a", "b", "c", "d"];
arr2.splice(1, 2);                // Remove 2 elements starting at index 1
console.log(arr2);                 // => ["a", "d"]

arr2.splice(1, 0, "x", "y");      // Insert at index 1 (remove 0)
console.log(arr2);                 // => ["a", "x", "y", "d"]

const fruits = ["apple", "banana", "cherry"];

fruits.forEach((fruit, index) => {
  console.log(`${index}: ${fruit}`);
});
// 0: apple
// 1: banana
// 2: cherry

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

// Practical: extract property from array of objects
const users = [
  { name: "Alice", age: 30 },
  { name: "Bob", age: 25 },
  { name: "Charlie", age: 35 }
];
const names = users.map(user => user.name);
console.log(names);  // => ["Alice", "Bob", "Charlie"]

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

// Practical: filter active users
const activeUsers = users.filter(user => user.age > 25);
console.log(activeUsers);  // => [{ name: "Alice", age: 30 }, { name: "Charlie", age: 35 }]

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

// Sum all numbers
const sum = numbers.reduce((total, current) => total + current, 0);
console.log(sum);  // => 15

// Find maximum
const max = numbers.reduce((max, n) => n > max ? n : max, numbers[0]);
console.log(max);  // => 5

// Count occurrences
const items = ["apple", "banana", "apple", "cherry", "banana", "apple"];
const counts = items.reduce((acc, item) => {
  acc[item] = (acc[item] || 0) + 1;
  return acc;
}, {});
console.log(counts);  // => { apple: 3, banana: 2, cherry: 1 }

// Copy
const original = [1, 2, 3];
const copy = [...original];
console.log(copy);  // => [1, 2, 3]

// Merge
const arr1 = [1, 2];
const arr2 = [3, 4];
const merged = [...arr1, ...arr2];
console.log(merged);  // => [1, 2, 3, 4]

// Add elements
const withInsert = [0, ...original, 4];
console.log(withInsert);  // => [0, 1, 2, 3, 4]

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

const [first, second] = colors;
console.log(first);   // => "red"
console.log(second);  // => "green"

// Skip elements
const [primary, , tertiary] = colors;
console.log(primary);   // => "red"
console.log(tertiary);  // => "blue"

// Rest pattern
const [head, ...rest] = colors;
console.log(head);  // => "red"
console.log(rest);  // => ["green", "blue", "yellow"]

// Default values
const [a = 1, b = 2, c = 3] = [10];
console.log(a, b, c);  // => 10, 2, 3

// join — array to string
const words = ["Hello", "World"];
console.log(words.join(" "));    // => "Hello World"
console.log(words.join("-"));    // => "Hello-World"

// split — string to array
const csv = "apple,banana,cherry";
console.log(csv.split(","));     // => ["apple", "banana", "cherry"]

const numbers = [3, 1, 10, 2, 20];
numbers.sort();                         // ❌ Sorts as strings: [1, 10, 2, 20, 3]
console.log(numbers);                    // => [1, 10, 2, 20, 3]

// Correct: with comparator
numbers.sort((a, b) => a - b);          // Ascending
console.log(numbers);                    // => [1, 2, 3, 10, 20]

numbers.sort((a, b) => b - a);          // Descending
console.log(numbers);                    // => [20, 10, 3, 2, 1]

// Sorting objects
const users = [
  { name: "Bob", age: 25 },
  { name: "Alice", age: 30 },
  { name: "Charlie", age: 20 }
];
users.sort((a, b) => a.age - b.age);    // Sort by age ascending
console.log(users.map(u => u.name));    // => ["Charlie", "Bob", "Alice"]

const matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
];

console.log(matrix[0][0]);  // => 1
console.log(matrix[1][2]);  // => 6
console.log(matrix[2][1]);  // => 8

// Iterate
for (const row of matrix) {
  for (const cell of row) {
    console.log(cell);
  }
}

// 1. Create an array of numbers and use map to double each
// 2. Use filter to get only positive numbers from [-3, 5, -1, 8, 0, 2]
// 3. Use reduce to find the product of [1, 2, 3, 4, 5]
// 4. Sort an array of objects by a string property
// 5. Flatten [[1,2], [3,4], [5,6]] into [1,2,3,4,5,6] using reduce

const nested = [[1,2], [3,4], [5,6]];
const flat = nested.reduce((acc, arr) => [...acc, ...arr], []);
console.log(flat);  // => [1, 2, 3, 4, 5, 6]