Arrays & objects

Most data work is transformation: take a collection and produce a new one. Three array methods cover the vast majority of cases. map transforms each item one-to-one. filter keeps the items that pass a test. reduce folds the whole array down into a single accumulated value — and it's the most general of the three.

const nums = [1, 2, 3, 4];
nums.map((n) => n * 2);        // [2, 4, 6, 8]
nums.filter((n) => n % 2 === 0); // [2, 4]
nums.reduce((sum, n) => sum + n, 0); // 10

Destructuring

Destructuring unpacks values into variables by position (arrays) or by key (objects). It makes function signatures and assignments read like the shape of the data.

const [first, second] = ["a", "b"];      // first="a", second="b"
const { name, age } = { name: "Ada", age: 36 };

function greet({ name }) {               // destructure a parameter
  return "Hi " + name;
}

Spread

The spread operator (...) expands an array or object inline — handy for copying, merging, and combining without mutating the originals.

const base = { theme: "dark" };
const next = { ...base, theme: "light", lang: "en" }; // new object
const combined = [...[1, 2], ...[3, 4]];               // [1, 2, 3, 4]

The challenge uses reduce to group people by their role into an object of arrays — a pattern you'll write constantly.

Next: how JavaScript handles work that doesn't finish right away.