Lesson 2 — Closures & private state

Functions remember

The scope of a variable is the part of the code that can see it. When a function is created inside another function, it keeps access to the outer function's variables even after the outer function has finished. That remembered link is called a closure.

function makeGreeter(word) {
  return (name) => word + ", " + name; // still sees word later
}
const hi = makeGreeter("Hi");
console.log(hi("Sam")); // Hi, Sam

Why it matters: closures give you private state that nothing outside can touch — counters, caches and event handlers all rely on them.