function* is not a gen.

xgen-and-gen.js

function* xcounter() {
  let index = 0;
  while (true) {
    index += 1;
    const x = yield index;
    console.log(x);
  }
}

const xgen = xcounter();
console.log(xgen.next().value);
console.log(xgen.next(42).value);
console.log(xgen.next().value);

function counter() {
  let index = 0;
  return () => {
    index += 1;
    return index;
  };
}

const gen = counter();
console.log(gen());
console.log(gen());
console.log(gen());