Microsoft® JScript™ ^= Operator 
Language Reference Version 1

Used to perform a bitwise exclusive OR on an expression.
result ^= expressionThe ^= operator syntax has these parts:
Part Description result Any variable. expression Any expression.
Using the ^= operator is exactly the same as specifying:
result = result ^ expressionThe ^= operator looks at the binary representation of the values of two expressions and does a bitwise exclusive OR operation on them. The result of this operation behaves as follows:
0101 (result) 1100 (expression)  1001 (result)When one, and only one, of the expressions has a 1 in a digit, the result has a 1 in that digit. Otherwise, the result has a 0 in that digit.
For information on when a runtime error is generated by the ^= operator, see the Operator Behavior table.