Generator

function* gen() { 
  yield 1;
  yield 2;
  yield 3;
}

var g = gen( // "Generator { }"

function* idMaker() {
    var index = 0;
    while(true)
        yield index++;
}

var gen = idMaker( // "Generator { }"

console.log(gen.next().value // 0
console.log(gen.next().value // 1
console.log(gen.next().value // 2
// ...

function fibonacci() {
  var a = yield 1;
  yield a * 2;
}

var it = fibonacci(
console.log(it          // "Generator {  }"
console.log(it.next()   // 1
console.log(it.send(10) // 20
console.log(it.close()  // undefined
console.log(it.next()   // throws StopIteration (as the generator is now closed)