fullAdder.js

const or = (a, b) => Boolean(a) || Boolean(b);
const orBit = toBit(or);

const fullAdder = (a, b, c = 0) => {
  const first = halfAdder(a, b);
  const second = halfAdder(first.s, c);
  return { c: orBit(first.c, second.c), s: second.s };
};

function addTwoBits(a1, a2) {
  // Note that we start with the index `1` here
  // because we need to start from the right-most (least significant) bit.
  // e.g.) given a1 = "01", we start from "1", not "0".
  const { c: c0, s: b0 } = halfAdder(+a1[1], +a2[1]);
  const { c: c1, s: b1 } = fullAdder(+a1[0], +a2[0], c0);
  return { c1, b1, b0 };
}

[
  ["00", "10"], ["01", "00"], ["10", "00"],
  ["10", "01"], ["10", "10"], ["11", "00"],
  ["11", "01"], ["11", "10"], ["11", "11"]
].map(v => log(`addTwoBits(${v[0]}, ${v[1]})`, addTwoBits(v[0], v[1])));

// result
// addTwoBits(00, 10) { c1: 0, b1: 1, b0: 0 }
// addTwoBits(01, 00) { c1: 0, b1: 0, b0: 1 }
// addTwoBits(10, 00) { c1: 0, b1: 1, b0: 0 }
// addTwoBits(10, 01) { c1: 0, b1: 1, b0: 1 }
// addTwoBits(10, 10) { c1: 1, b1: 0, b0: 0 }
// addTwoBits(11, 00) { c1: 0, b1: 1, b0: 1 }
// addTwoBits(11, 01) { c1: 1, b1: 0, b0: 0 }
// addTwoBits(11, 10) { c1: 1, b1: 0, b0: 1 }
// addTwoBits(11, 11) { c1: 1, b1: 1, b0: 0 }