es6 generator函數

yiled 表達式的返回值

function* foo() {
  yield new Promise((resolve, reject) => {
    resolve();
  });
  const result = yield console.log(0);
  result.then(() => { console.log(2); });
  console.log(1);
}
const tt = foo();
const result = tt.next();   
console.log(result);   //{value: Promise, done: false}
console.log(tt.next()); 
// 0
//{value: undefined, done: false}
tt.next(result.value);   
// 1
// 2
tt.next(); //{value: undefined, done: true}