async函数转为 generator 函数，并放入co自执行函数中运行，await就相当于generator函数中的 yield 执行器，执行一次promise 返回{value, done} 

    1. function yieldPromise(generator){
    2.     var iterator=generator();
    3.     recursiveCore.call(iterator);
    4. }
    6. function recursiveCore(feedback){
    7.     var iterator=this,
    8.         result=iterator.next(feedback);
    10.     if(result.done){
    11.         return;
    12.     }
    14.     var promise=result.value;
    15.     Promise.resolve(promise).then(function(v){
    16.         recursiveCore.call(iterator,v);
    17.     });
    18. }
    21. yieldPromise(function*(){
    22.     var v1=yield new Promise(function(resolve,reject){
    23.         setTimeout(function(){
    24.             resolve('Hello');
    25.         },1500);
    26.     });
    28.     console.warn(v1);
    30.     var v2=yield new Promise(function(resolve,reject){
    31.         setTimeout(function(){
    32.             resolve('World');
    33.         },1500);
    34.     });
    36.     console.warn(v2);
    37. });
    1. function yeildPromise(gen) {
    2.     const iterator = gen()
    3.   const next = data => {
    4.       const { done, value } = iterator.next(data);
    6.     if(done) return value;
    8.     value.then(data => next(data))
    9.   }
    11.   next();
    12. }
    1. function asyncToGenerator(generatorFunc) {
    2.     return function() {
    3.       const gen = generatorFunc.apply(this, arguments)
    4.       return new Promise((resolve, reject) => {
    5.         function step(key, arg) {
    6.           let generatorResult
    7.           try {
    8.             generatorResult = gen[key](arg)
    9.           } catch (error) {
    10.             return reject(error)
    11.           }
    12.           const { value, done } = generatorResult
    13.           if (done) {
    14.             return resolve(value)
    15.           } else {
    16.             return Promise.resolve(value).then(val => step('next', val), err => step('throw', err))
    17.           }
    18.         }
    19.         step("next")
    20.       })
    21.     }
    22. }
    1. const obj = {a: 'a1', b: 'b1', c: 'c1'};
    2. // typeof obj[Symbol.iterator] // 'undefined'
    3. //
    4. // for (const val of obj) {
    5. //   console.log(val);
    6. // }
    7. // VM974:4 Uncaught TypeError: obj is not iterable
    9. obj[Symbol.iterator] = function() {
    10.   const self = this;
    11.   const keys = Object.keys(this);
    12.   const len = keys.length;
    13.   let pointer = 0;
    14.   return {
    15.     next() {
    16.       const done = pointer >= len;
    17.       console.log('----', keys);
    18.       const value = !done ? self[keys[pointer++]] : undefined;
    19.       return {
    20.         value,
    21.         done
    22.       };
    23.     }
    24.   }
    25. }
    26. for (const val of obj) {
    27.   console.log('val', val);
    28. }
    29. // 'a1'
    30. // 'b1'
    31. // 'c1'