https://es6.ruanyifeng.com/#docs/async#async-%E5%87%BD%E6%95%B0%E7%9A%84%E5%AE%9E%E7%8E%B0%E5%8E%9F%E7%90%86

    1. async function fn(args) {
    2.   // ...
    3. }
    5. // 等同于
    7. function fn(args) {
    8.   return spawn(function* () {
    9.     // ...
    10.   });
    11. }
    13. // 具体实现
    14. function spawn(genF) {
    15.   return new Promise(function(resolve, reject) {
    16.     const gen = genF();
    17.     function step(nextF) {
    18.       let next;
    19.       try {
    20.         next = nextF();
    21.       } catch(e) {
    22.         return reject(e);
    23.       }
    24.       if(next.done) {
    25.         return resolve(next.value);
    26.       }
    27.       Promise.resolve(next.value).then(function(v) {
    28.         step(function() { return gen.next(v); });
    29.       }, function(e) {
    30.         step(function() { return gen.throw(e); });
    31.       });
    32.     }
    33.     step(function() { return gen.next(undefined); });
    34.   });
    35. }