As we know JK flip flop behaves in the same fashion as an RS flip flop except for one of the entries in the function table. But RS flip flop has big drawback at the input combination S = R = 1 (in the case of a flip-flop with active HIGH inputs) and the input combination S = R = 0 (in the case of a flip-flop with active LOW inputs) are prohibited. For know the working principle of RS flip flop you may follow my previous post RS flip flop. So overcome the situation we need some new one. In the case of a JK flip flop with active HIGH inputs, the output of the flip-flop toggles, that is, it goes to the other state, for J = K = 1 . The output toggles for J = K = 0 in the case of the flip-flop having active LOW inputs. So we can say JK flip flop is invented for overcome the problem of SR flip flop.
In bellow you just see the block diagram of JK flip flop with truth table.
Now by using two AND gate with a RS flip flop we can realize JK flip flop. See in bellow the realization of JK flip flop using RS flip flop.