2adder.js

// Two-bit binary adder (manual chaining of two full adders)
let a = [1, 1];      
let b = [1, 1];      
let carry = 0;       // initial carry-in
let result = [];     // will hold the sum bits


let sumLSB = a[1] ^ b[1] ^ carry;                // sum = a xor b xor carry-in
let carryOut = (a[1] & b[1]) | (b[1] & carry) | (a[1] & carry); // carry-out
result[1] = sumLSB;      // store LSB in result[1]
carry = carryOut;        // propagate carry

let sumMSB = a[0] ^ b[0] ^ carry;  
let carryOut2 = (a[0] & b[0]) | (b[0] & carry) | (a[0] & carry);
result[0] = sumMSB;      // store MSB in result[0]
carry = carryOut2;       // final carry after most significant bit

// If final carry is 1, we need an extra most significant bit
if (carry === 1) {
    result.unshift(1);
}

// Display the result
console.log('  ', a);
console.log('  ', b);
console.log('------------');
console.log(result);