Functions, Arrays and Objects

Functions

function add(a, b) {
  return a + b;
}

const multiply = (a, b) => a * b;          // arrow function
const square = n => n * n;                 // one parameter, implicit return

function greet(name = "friend") {          // default parameter
  return `Hello, ${name}!`;
}

function sum(...numbers) {                 // rest parameter
  return numbers.reduce((acc, n) => acc + n, 0);
}

Functions are values: you can store them, pass them and return them.

Arrays

const tasks = ["plan", "build"];
tasks.push("ship");            // add to end
tasks.includes("build");       // true
tasks.indexOf("ship");         // 2
tasks.length;

The three methods you will use constantly:

const products = [
  { name: "Hoodie", cents: 3900, inStock: true },
  { name: "Mug", cents: 1250, inStock: false },
  { name: "Cap", cents: 1800, inStock: true },
];

const names = products.map(p => p.name);                    // transform
const available = products.filter(p => p.inStock);          // select
const totalCents = products.reduce((sum, p) => sum + p.cents, 0); // combine

const cheapest = [...products].sort((a, b) => a.cents - b.cents)[0];
const hasMug = products.some(p => p.name === "Mug");
const found = products.find(p => p.cents > 3000);

sort changes the original array, so copy it first with [...array] (or use toSorted() in modern browsers).

Objects

const user = {
  name: "Ada",
  role: "admin",
  greet() { return `Hi, I am ${this.name}`; },
};
user.email = "ada@example.com";
user["role"];
Object.keys(user);
Object.entries(user).forEach(([key, value]) => console.log(key, value));

Destructuring and spread

const { name, role = "user" } = user;          // pull out properties
const [first, ...rest] = tasks;                 // pull out array items

const updated = { ...user, role: "editor" };    // copy with a change
const allTasks = [...tasks, "celebrate"];       // copy with an addition

Spreading creates shallow copies. Nested objects are still shared; use structuredClone(obj) for a deep copy.

JSON

const text = JSON.stringify({ id: 1, tags: ["js"] });   // to string
const data = JSON.parse(text);                           // back to object

Exercise

Given an array of order objects ({ id, customer, items: [{ cents, qty }] }), compute each order's total with map and reduce, keep orders over $50 with filter, and log the customer names sorted alphabetically.

Check yourself

• What does map return compared with forEach?
• Why copy an array before calling sort?
• What does { ...user, role: "editor" } create?