TypeScript Practice 11/12

Generic constraints

Lesson 5's first<T> worked for any type because its body never touched the elements - arr[0] needs nothing from T. But most useful generic functions do need something: a function picking the most recently updated item must read .updated - and an unconstrained T promises nothing, so TypeScript refuses the read.

The middle ground between "any type" and "one exact type" is a constraint:

function newest<T extends { updated: number }>(items: T[]): T {

T extends { updated: number } reads as: T can be anything, as long as it has a numeric updated. Blog posts, files, user records - all welcome, each keeping its own full type. The payoff is in the return: callers get back their complete item type, not some stripped-down { updated: number } - newest(posts).title works, because T remembered it was a post all along.

Your task: implement newest(items): given a non-empty array of items that each have a numeric updated timestamp, return the item with the largest updated.

You'll practice:

Writing T extends { ... } constraints
Relying on the constrained capability inside the body while preserving the full type

// Return the item with the largest .updated - for ANY item type that has one.
function newest<T extends { updated: number }>(items: T[]): T {

}

const posts = [
  { title: "Intro", updated: 100 },
  { title: "Deep dive", updated: 300 },
  { title: "Errata", updated: 200 },
];
const files = [
  { path: "/a.txt", size: 10, updated: 5 },
  { path: "/b.txt", size: 20, updated: 9 },
];

const newestPost = newest(posts);
const newestFile = newest(files);