Question

I am using bitwise operations in order to represent many access control flags within one integer.

ADMIN_ACCESS = 1;
EDIT_ACCOUNT_ACCESS = 2;
EDIT_ORDER_ACCESS = 4;

var myAccess = 3; // ie: ( ADMIN_ACCESS | EDIT_ACCOUNT_ACCESS )

if ( myAccess & EDIT_ACCOUNT_ACCESS ) { // check for correct access
   // allow for editing of account

}

Most of this is occurring on the PHP side of my project. There is one piece however where Javascript is used to join several access flags using | when saving someone’s access level. This works fine to a point. I have found that once an integer (flag) gets too large (> 32bit), it no longer works correctly with bitwise operators in Javascript. For instance:

alert( 4294967296 | 1 ); // equals 1, but should equal 4294967297

I am trying to find a workaround for this so that I do not have to limit my number of access control flags to 32. Each access control flag is two times the previous control flag so that each control flag will not interfere with other control flags.

dec(4) = bin(100)
dec(8) = bin(1000)
dec(16) = bin(10000)

I have noticed that when adding two of these flags together with a simple +, it seems to come out with the same answer as a bitwise or operation, but am having trouble wrapping my head around whether this is a simple substitution, or if there might be problems with doing this. Can anyone comment on the validity of this workaround? Example:

(4294967296 | 262144 | 524288) == (4294967296 + 262144 + 524288)

Answer 1

Simply adding the flags will work provided you know for sure that each flag is a power of two, and that you don’t go past 52 bits long (due to the amount a Double-precision float can hold, since that’s what JS uses for numbers).

If, for whatever reason, you need more than 52 flags, I would suggest grouping the flags into categories.