Array
Toolbox

// forEach: do something with every item. It gives nothing back.
const prices = [30, 80, 10];
prices.forEach((price, index) => console.log(`item ${index} costs ₹${price}`));

// map: build a NEW array by changing every item
const withDelivery = prices.map((price) => price + 5);
console.log(withDelivery);
console.log(prices); // the original stays the same

const cart = [
  { name: "apple", price: 30, qty: 4 },
  { name: "mango", price: 80, qty: 2 },
  { name: "banana", price: 10, qty: 12 },
];

// pull one field out of every object
const names = cart.map((item) => item.name);
console.log(names);

// turn every object into a label
const labels = cart.map((item) => `${item.qty} x ${item.name}`);
console.log(labels);

const cart = [
  { name: "apple", price: 30 },
  { name: "mango", price: 80 },
  { name: "banana", price: 10 },
];

const cheap = cart.filter((item) => item.price < 50);        // ALL that match
console.log(cheap.map((item) => item.name));

const mango = cart.find((item) => item.name === "mango");     // FIRST that matches
console.log(mango);

console.log(cart.findIndex((item) => item.name === "banana")); // its position
console.log(cart.find((item) => item.name === "kiwi"));       // nothing found

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

console.log(fruits.includes("mango"));     // is it there at all?
console.log(fruits.indexOf("mango"));      // first position
console.log(fruits.lastIndexOf("mango"));  // last position
console.log(fruits.indexOf("kiwi"));       // -1 means "not found"

console.log(fruits.at(0), fruits.at(-1));  // first item, last item

const ages = [19, 24, 17, 31];

console.log(ages.some((age) => age < 18));    // is at least ONE under 18?
console.log(ages.every((age) => age >= 18));  // is EVERY one 18 or older?

// both stop early: some at the first yes, every at the first no
console.log([].some((n) => n > 0), [].every((n) => n > 0)); // empty array

const prices = [30, 80, 10];

// reduce(helper, startValue)
// the helper gets the running result and the current item,
// and returns the new running result
const total = prices.reduce((sum, price) => sum + price, 0);
console.log(total);

// the same thing written as a plain loop
let sum = 0;
for (const price of prices) sum = sum + price;
console.log(sum);

const cart = [
  { name: "apple", price: 30, qty: 4 },
  { name: "mango", price: 80, qty: 2 },
  { name: "banana", price: 10, qty: 12 },
];

// 1. the full bill
const bill = cart.reduce((sum, item) => sum + item.price * item.qty, 0);
console.log(bill);

// 2. count how often each vote appears
const votes = ["yes", "no", "yes", "yes", "maybe"];
const tally = votes.reduce((count, vote) => {
  count[vote] = (count[vote] ?? 0) + 1;
  return count;
}, {});
console.log(tally);

// 3. keep the most expensive item
const priciest = cart.reduce((best, item) => (item.price > best.price ? item : best));
console.log(priciest.name);

const queue = ["Asha", "Ravi"];

queue.push("Meera");    // add to the END
queue.unshift("Dev");   // add to the START
console.log(queue);

const last = queue.pop();    // remove from the END
const first = queue.shift(); // remove from the START
console.log(first, last);
console.log(queue);

const days = ["Mon", "Tue", "Wed", "Thu", "Fri"];

// slice(start, end): COPY a piece. The end index is not included.
console.log(days.slice(1, 3));
console.log(days.slice(-2));   // the last two
console.log(days);             // untouched

// splice(start, howMany, ...newItems): CUT from the real array
const removed = days.splice(1, 2, "Holiday");
console.log(removed);
console.log(days);             // changed!

const nums = [10, 1, 5, 100];

// careful: without a helper, sort compares items as text
console.log([...nums].sort());

// (a, b) => a - b means small to big, b - a means big to small
console.log([...nums].sort((a, b) => a - b));
console.log(nums.toSorted((a, b) => b - a)); // new array, nums unchanged
console.log(nums);

const words = ["mango", "Apple", "banana"];
console.log(words.toSorted((a, b) => a.localeCompare(b)));

const steps = ["wake", "brush", "breakfast"];

console.log(steps.toReversed());   // new, reversed copy
console.log(steps.with(1, "shower")); // new copy with one item swapped
console.log(steps);                // still the same

steps.reverse();                   // reverse changes the original
console.log(steps);

const nested = [1, [2, 3], [4, [5]]];
console.log(nested.flat());          // open one level of boxes
console.log(nested.flat(Infinity));  // open every level

const lines = ["good morning", "see you soon"];
console.log(lines.flatMap((line) => line.split(" "))); // map, then flat

console.log(["2026", "09", "28"].join("-"));  // array to text
console.log([1, 2].concat([3, 4], 5));        // glue arrays together

// make arrays from scratch
console.log(Array.from({ length: 5 }, (_, i) => i + 1)); // 1 to 5
console.log(new Array(3).fill("-"));                      // three dashes
console.log(Array.from("hey"));                           // text to letters
console.log(Array.from(new Set([1, 1, 2, 3, 3])));        // remove repeats

// check what you have
console.log(Array.isArray([1, 2]), Array.isArray("12"));
console.log([4, 5, 6].length);

const orders = [
  { customer: "Asha", items: ["apple", "mango"], total: 110, paid: true },
  { customer: "Ravi", items: ["banana"], total: 120, paid: false },
  { customer: "Meera", items: ["mango", "kiwi", "apple"], total: 260, paid: true },
  { customer: "Dev", items: ["kiwi"], total: 90, paid: true },
];

const paid = orders.filter((o) => o.paid);                        // keep paid ones
const revenue = paid.reduce((sum, o) => sum + o.total, 0);        // add them up
const top2 = paid
  .toSorted((a, b) => b.total - a.total)                          // biggest first
  .slice(0, 2)                                                    // take two
  .map((o) => o.customer);                                        // just names
const fruits = [...new Set(orders.flatMap((o) => o.items))].sort(); // every fruit once
const anyUnpaid = orders.some((o) => !o.paid);

console.log("revenue:", revenue);
console.log("top customers:", top2);
console.log("fruits sold:", fruits.join(", "));
console.log("chase payments?", anyUnpaid);