"&","&&";"|","||"的用法(整理)
参考：http://developers-heaven.net/forum/index.php?topic=56.0
http://www.javaeye.com/topic/462449
从下面那篇文章中的(i & 1) == 0说起，当i的对位与的值为1时，比如1,3，5，i&1返回是1，否则返回是0(比如2,4,6)。
再看以下需求：
成长速度为15显示4个箭头；
成长速度为12显示3个箭头；
成长速度为10显示2个箭头；
成长速度为5显示1个箭头；
成长速度为0显示0个箭头。
用&&和||替代if及swith...case的写法：
var add_level = (add_step==5 && 1) || (add_step==10 && 2) || (add_step==12 && 3) ||
(add_step==15 && 4) || 0;
更简洁的写法：
var add_level={'5':1,'10':2,'12':3,'15':4}[add_step] || 0;
请你一定要记住：在js逻辑运算中，0、""、null、false、undefined、NaN都会判为false，其他都为true。这个一定要记住，不然应用||和&&就会出现问题。
这里顺便提下：看到很多代码if(!!attr)，为什么不直接写if(attr)；
其实这是一种更严谨的写法：
请测试 typeof 5和typeof !!5的区别。!!的作用是把一个其他类型的变量转成的bool类型。
另：
1 - In the conditional statements:
just like the & and && operator, the double Operator || is a "short-circuit" operator
For example:
if(condition1 || condition 2 || condition 3)
If condition1 is true, condition 2 and 3 will NOT be checked.
if(condition1 | condition 2 | condition 3)
This will check conditions 2 and 3, even if 1 is already true.
2- There is a Second use of the | and & operator though: Bitwise Operations.
|
is the bitwise OR operator. Its used to operate on two numbers. You
look at each bit of each number individually and, if one of the bits is 1
in at least one of the numbers, then the resulting bit will be 1 also.
Here are a few examples:
A = 01010101
B = 10101010
A | B = 11111111
A & B = 00000000
A = 00000001
B = 00010000
A | B = 00010001
A & B = 00000000
A = 10001011
B = 00101100
A | B = 10101111
A & B = 00001000
