What Is Closure?

A closure happens when a function remembers variables from its outer function/scope even after the outer function has finished executing.

Example:

function counter() {
  let count = 0;        // local variable

  return function () {  // inner function
    count++;            // still accessing the outer variable
    console.log(count);
  };
}

let c = counter(); // counter() is done executing now
c(); // 1
c(); // 2
c(); // 3

Why does c() still increment count when counter() has already finished?
Because the inner function kept a reference to the variable count.
That preserved scope = closure.