function* f() {}

function* helloWorldGenerator() {
 yield 'hello';
 yield 'world';
 return 'ending';
}
var hw = helloWorldGenerator();//第一次调用该方法不会执行，仅返回一个遍历器。
var a = hw.next();
while(!a.done){ //当执行到return 时，a.done=true,终止循环
 console.log(a.value+','+a.done);
 a = hw.next();
}
console.log(a.value+','+a.done);

hello,false
world,false
ending,true

function* f() {
 for(var i=0; true; i++) {
  var reset = yield i;
  console.log(reset); //打印reset，验证yield语句是没有返回值的
  if(reset) { i = -1; }
 }
}
var g = f();
console.log(g.next()) // { value: 0, done: false }
console.log(g.next())// { value: 1, done: false }
console.log(g.next(true)) // { value: 0, done: false }

{ value: 0, done: false }
undefined
{ value: 1, done: false }
true
{ value: 0, done: false }

function* foo(x) {
 var y = 2 * (yield (x + 1));
 var z = yield (y / 3);
 return (x + y + z);
}
var it = foo(5);
console.log(it.next(3))// { value:6, done:false }
console.log(it.next(12))// { value:8, done:false }
console.log(it.next(13))// { value:42, done:true }

function *foo() {
 yield 1;
 yield 2;
 yield 3;
 yield 4;
 yield 5;
 return 6;
}
for (let v of foo()) {
 console.log(v);
}
// 1 2 3 4 5

let a = (function* () {
 yield 'Hello!';
 yield 'Bye!';
}());
let b = (function* () {
 yield 'Greetings!';
 yield* a;
 yield 'Ok, bye.';
}());
for(let value of b) {
 console.log(value);
}

Greetings!
Hello!
Bye!
Ok, bye.

function* gen(){
 yield* ["a", "b", "c"];
}
gen().next() // { value:"a", done:false }

function* iterTree(tree) {
 if (Array.isArray(tree)) {
 for(let i=0; i < tree.length; i++) {
  yield* iterTree(tree[i]);
 }
 } else {
 yield tree;
 }
}
const tree = [ 'a', ['b', 'c'], ['d', 'e'] ];
for(let x of iterTree(tree)) {
 console.log(x);
}// a b c d e