JavaScript里的循环方法：forEach，for-in，for-of: http://www.webhek.com/post/javascript-loop-foreach-for-in-for-of.html

for...of - JavaScript | MDN - Mozilla

深入浅出ES6（二）：迭代器和for-of循环: http://www.infoq.com/cn/articles/es6-in-depth-iterators-and-the-for-of-loop

javascript总for of和for in的区别？_segmentfault: https://segmentfault.com/q/1010000006658882

javascript for...in vs for: https://stackoverflow.com/questions/242841/javascript-for-in-vs-for

深入解析 ES6：Iterator 和 for-of 循环: http://bubkoo.com/2015/06/15/es6-in-depth-iterators-and-the-for-of-loop/

“foreach” vs “for of” vs “for in” in JavaScript: http://qnimate.com/foreach-vs-for-of-vs-for-in-in-javascript/

MDN Web文档: https://developer.mozilla.org/zh-CN/