What does the operator |= do in JavaScript?

Question

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; 
}

What does the |= do in the body of the for loop?

The code example is dubious, but has been presented here by V8 for an example of improved performance.

Updated Example

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
}

Answer 1

a[0] |= b

is basically

a[0] = a[0] | b

"|" is an or bitwise operator

Update When a[0] is assigned 0, a[0] in binary is 0000. In the loop,

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)
    

At the end of the loop the value of a[0] is 15