运算符| =在JavaScript中做什么？ [英] What does the operator |= do in JavaScript?
问题描述
我在JS项目中找到了以下代码：
I found the following code in a JS project:
var a = new Array();
a[0] = 0;
for (var b = 0; b < 10; b++) {
a[0] |= b;
}
| = 在for循环体中做什么？
What does the |= do in the body of the for loop?
代码示例可疑，但已经提交这里由V8提供改进性能的示例。
The code example is dubious, but has been presented here by V8 for an example of improved performance.
更新示例
上述示例相当于 var a = [15]; 对于大多数意图和目的。 | = 运算符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：
The above example is equivalent to var a = [15]; for most intents and purposes. A more realistic example for the |= operator would be to set up binary flags in a single variable, for example on a permission object:
//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
}
推荐答案
a[0] |= b
基本上是
a[0] = a[0] | b
|是一个或按位运算符（顺便说一下： MDN文档写得非常好，非常清楚。如果OP想要编写和理解JS，那么MDN文档就是一个很好的资源。）
"|" is an or bitwise operator (by the way: The MDN docs are really well written and really clear. If the OP is wanting to write and understand JS, then the MDN docs are a great resource.)
更新
当 a [0] 被分配 0 时， a二进制中的[0] 是 0000 。在循环中，
Update
When a[0] is assigned 0, a[0] in binary is 0000. In the loop,
-
b = 0
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)
b = 1
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)
b = 2
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)
b = 3
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)
b = 4
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)
b = 5
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)
b = 6
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)
b = 7
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)
b = 8
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)
b = 9
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)
在循环结束时， a [0] 的值为 15
At the end of the loop the value of a[0] is 15
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