Challenge 2 — Solution

Task: Write a function makeIdGenerator() that returns a function
with no parameters, returning a new sequential ID each time it's
called (starting at 1). Create two SEPARATE generators and call
each one a few times to prove their counts don't interfere with
each other.

function makeIdGenerator() {
    let nextId = 1;
    return function () {
        const id = nextId;
        nextId++;
        return id;
    };
}

const generatorA = makeIdGenerator();
const generatorB = makeIdGenerator();

console.log(generatorA());   // 1
console.log(generatorA());   // 2
console.log(generatorB());   // 1
console.log(generatorA());   // 3
console.log(generatorB());   // 2


Expected output:

1
2
1
3
2


Notes:
- generatorA and generatorB each came from their own separate call
  to makeIdGenerator(), so each one closes over its own private
  nextId variable — generatorB starting at 1 again, completely
  unaffected by generatorA already being at 2.
- Interleaving the calls (A, A, B, A, B) demonstrates the two
  counters are genuinely independent, not just coincidentally
  starting from the same number.
- nextId is never accessible from outside either generator function —
  there's no way to read or reset it directly, only by calling the
  generator itself.