It is the bitwise XOR operator. From the MDN docs:
[Bitwise XOR] returns a one in each bit position for which the corresponding bits of
either but not both operands are ones.
Where the operands are whatever is on the left or right of the operator.
For example, if we have two bytes:
A 11001100
B 10101010
We end up with
Q 01100110
If a bit in A is set OR a bit in B is set, but NOT both, then the result is a 1, otherwise it is 0.
In the example you give, it will take the binary representation of the ASCII character code from d.charCodeAt(i) and k.charCodeAt(i) and XOR them. It does the same in Python, C++ and most other languages. It is not to be confused with the exponential operator in maths-related contexts; languages will provide a pow() function or similar. JavaScript for one has Math.pow(base, exponent).