[javascript] 判斷變數 null、undefined及空值

1. 判斷變數為 null 或 undefined 或 ""

   [javascript] if(!str) { var str; console.log(“str is null or undefined or ” ”); } [/javascript]

2. 判斷變數是否存在

   [javascript] if(typeof str === “undefined”) console.log(“str is not exist”); [/javascript]

   PS. === 可以辨別型態， == 則會轉型單純比值

參考資料：

http://www.ruanyifeng.com/blog/2011/05/how_to_judge_the_existence_of_a_global_object_in_javascript.html