[js] 2-bit counter with JK flip-flops

jk-flipflop.js

function flip(bit) {
    return bit ? 0 : 1;
}

// each call simulates clock tick
function jk() {
    var q=0;
    return function(j, k)  {
        if (j) {
            if (k)
                // // Q(t+1) = ~Q(t))
                return (q = flip(q));
            else
                // // Q(t+1) = 1
                return (q = 1);
        } else {
            if (k)
                // // Q(t+1) = 0
                return (q = 0);
            else
                // Q(t+1) = Q(t)
                return q;
        }
    };
}

/*
 * JK-flip flop demo - 2-bit counter
 * each call simulates clock tick
 * sample run:
 *  > c1 = counter()
 * => [Function]
 *  > c1()
 * => [ 0, 0 ]
 *  > c1()
 * => [ 0, 1 ]
 *  > c1()
 * => [ 1, 0 ]
 *  >  c1()
 * => [ 1, 1 ]
 *  > c1()
 * => [ 0, 0 ]
 */
function counter() {
    var ff1=jk();    //MSB
    var ff0=jk();    //LSB
    var a1 = 0, a0 = 0;
    return function() {
        var r = [a1, a0];
        a1 = ff1(a0, a0);
        a0 = ff0(1, 1);
        return r;
    };
}