[Javascript] 檢查json的key是否存在

太常用到了，記錄一下。使用hasOwnProperty就可以輕鬆解決嚕 wow

You don't need jQuery for this, just JavaScript. You can do it a few ways:

• typeof d.failed- returns the type ('undefined', 'Number', etc)
• d.hasOwnProperty('failed')- just in case it's inherited
• 'failed' in d- check if it was ever set (even to undefined)