运算符| =在JavaScript中做什么？

Question

我在JS项目中找到了以下代码:

var a = new Array();
a[0] = 0;
for (var b = 0; b < 10; b++) {
  a[0] |= b; 
}

|=for循环体内有什么作用？

代码示例是可疑的,但V8 已经在这里提供了一个改进性能的示例.

更新的示例

上面的例子相当于var a = [15];大多数意图和目的.|=操作员的一个更现实的例子是在单个变量中设置二进制标志,例如在权限对象上:

//Set up permission masks
var PERMISSION_1_MASK = parseInt('0001',2);
var PERMISSION_2_MASK = parseInt('0010',2);
..

//Set up permissions
userPermissions = 0;
userPermissions |= hasPermissionOne && PERMISSION_1_MASK;
userPermissions |= hasPermissionTwo && PERMISSION_2_MASK;
..

//Use permissions
if(userPermissions & PERMISSION_1_MASK){
    ..//Do stuff only allowed by permission 1
}

Answer 1

a[0] |= b

基本上是

a[0] = a[0] | b

"|" 是一个或按位运算符

更新 何时a[0]分配0,a[0]二进制是0000.在循环,

1. b = 0

   a[0] = 0 (base 10) = 0000 (base 2)
   b    = 0 (base 10) = 0000 (base 2)
                      ---------------
   a[0] | b           = 0000 (base 2) = 0 (base 10)
   

2. b = 1

   a[0] = 0 (base 10) = 0000 (base 2)
   b    = 1 (base 10) = 0001 (base 2)
                      ---------------
   a[0] | b           = 0001 (base 2) = 1 (base 10)
   

3. b = 2

   a[0] = 1 (base 10) = 0001 (base 2)
   b    = 2 (base 10) = 0010 (base 2)
                      ---------------
   a[0] | b           = 0011 (base 2) = 3 (base 10)
   

4. b = 3

   a[0] = 3 (base 10) = 0011 (base 2)
   b    = 3 (base 10) = 0011 (base 2)
                      ---------------
   a[0] | b           = 0011 (base 2) = 3 (base 10)
   

5. b = 4

   a[0] = 3 (base 10) = 0011 (base 2)
   b    = 4 (base 10) = 0100 (base 2)
                      ---------------
   a[0] | b           = 0111 (base 2) = 7 (base 10)
   

6. b = 5

   a[0] = 7 (base 10) = 0111 (base 2)
   b    = 5 (base 10) = 0101 (base 2)
                      ---------------
   a[0] | b           = 0111 (base 2) = 7 (base 10)
   

7. b = 6

   a[0] = 7 (base 10) = 0111 (base 2)
   b    = 6 (base 10) = 0110 (base 2)
                      ---------------
   a[0] | b           = 0111 (base 2) = 7 (base 10)
   

8. b = 7

   a[0] = 7 (base 10) = 0111 (base 2)
   b    = 7 (base 10) = 0111 (base 2)
                      ---------------
   a[0] | b           = 0111 (base 2) = 7 (base 10)
   

9. b = 8

   a[0] = 7 (base 10) = 0111 (base 2)
   b    = 8 (base 10) = 1000 (base 2)
                      ---------------
   a[0] | b           = 1111 (base 2) = 15 (base 10)
   

10. b = 9

    a[0] = 15 (base 10) = 1111 (base 2)
    b    =  9 (base 10) = 1001 (base 2)
                        ---------------
    a[0] | b            = 1111 (base 2) = 15 (base 10)
    

在循环的最后的值a[0]是15

Answer 2

x |= y;

相当于

x = x | y;

其中|代表按位OR.

Answer 3

与大多数赋值运算符一样,它等同于再次使用左手值应用运算符:

a |= b
a = a | b

就像

a += b
a = a + b

查看Moz Dev Net了解更多信息.

[编辑:大脑失败,混淆了| 和||.需要更多咖啡.修改如下]

由于|是Bitwise OR运算符,结果a|b将是表示带有所有1位a和的bitstring的整数b.需要注意的是JavaScript有没有本地int或位串类型,所以它会先投a和b为int,然后做一个位或对位.所以9 | 二进制2是1001 | 0010 = 1011,即11,但是8 | 2 = 8.

其效果是增加的标志位b进入a.所以,如果你有一些旗帜WEEVILFLAG=parseInt(00001000,2):

// a = parseInt(01100001,2)
if(isWeevilish(a))
    a |= WEEVILFLAG;
// now a = parseInt(01101001,2)

将该位设置为1.

Answer 4

在每个位位置返回一个位,其中一个或两个操作数的相应位是1.

代码:result = a | b;

^是按位XOR运算符,它为每个位置返回一个运算符,其中一个(不是两个)其操作数的相应位是1.下一个示例返回4(0100):