对象判断总结

 1.非空判断

1)变量是null,undefined,0,"";if判断都为false
如：var t = null;
t? true: false;   // false 
2)变量是个空字符串，不为null
var t = "";
t ? true: false;   //false
t != null;   // true
3)变量是多个空格
var t = "     ";
t ? true: false;   //true
t != null;   // true
(t && t != null && $.trim(t) != "")? true: false;    //false
4)0和''比较
0 == '';   //true
0 === '';  //false

 2.对象判断

var t = {"a":1,"b":2,"c":3};
"a" in t;      //true
"d" in t;     //false