async 函数的实现原理，就是将 Generator 函数和自动执行器，包装在一个函数里 ``` function spawn(genF) { return new Promise(function(resolve, reject) { const gen = genF(); function step(nextF) { let next; try { next = nextF(); } catch (e) { return reject(e); } if (next.done) { return resolve(next.value); } Promise.resolve(next.value).then( function(v) { step(function() { return gen.next(v); }); }, function(e) { step(function() { return gen.throw(e); }); } ); } step(function() { return gen.next(undefined); }); }); } ```