Generator

function *testGen() {
  console.log('print some');
  yield 1;
  yield 2;
  yield 3;
}

const it = testGen();
it.next();  //1  { value: 1, done: false }
it.next();  //2  { value: 2, done: false }
it.next();  //3  { value: 3, done: false }
it.next();  //undefined { value: undefined, done: false }