SOURCE

function addBase36(num1,num2){
    const decimal1=parseInt(num1,36);
    const decimal2 = parseInt(num2,36);

    const sumDecimal = decimal1+decimal2;

    const base36Sum = sumDecimal.toString(36).toUpperCase();

    return base36Sum;
}

console.log(addBase36('A', 'B'))
console.log(addBase36('Z','1'))
console.log(addBase36('1A','2B'))