Lesson 3 — Promises

A value that arrives later

Some work takes time — a timer, a download. A Promise is an object that stands for a value you will get later. You create one with new Promise((resolve) => …) and call resolve(value) when the value is ready; .then(callback) runs once it is.

const later = new Promise((resolve) => {
  setTimeout(() => resolve(42), 100); // after 100 ms
});
later.then((v) => console.log(v)); // 42

Why it matters: every network request in a web app (fetch) gives you a Promise, so this is the base of all async code.