活用 es6 generator + co 讓async看起來像sync

仔細研究了es6的generator function + [co](https://github.com/tj/co "co") 才發現可以利用它來改善我們的程式寫法 讓程式的可讀性更高 先讓我們看看用Promise來撰寫非同步code的樣子 ```javascript let num = function (num) { return new Promise((resolve, reject) => { setTimeout(() => { resolve(num+1); }, 500); }); } num(1).then((res) => { return num(res); }).then((res) => { return num(res); }).then((res) => { // output // promise result: 4 console.log(`promise result: ${res}`); }); ``` 接下來讓我們來看看generator function + [co](https://github.com/tj/co "co")的寫法 ```javascript let num = function (num) { return (callback) => { setTimeout(function() { callback(null, num+1); }, 500); } } co(function*() { let num1 = yield num(1); let num2 = yield num(num1); let num3 = yield num(num2); return num3; }).then((res) => { // output // generator result: 4 console.log(`generator result: ${res}`); }); ``` [jsfiddle](https://jsfiddle.net/recca0120/eocp5h2c/ "jsfiddle範例")