Calculate the sum of large numbers.

sumBigNumber.js

/**
 * @param a {String}
 * @param b {String}
 */
function sumBigNumber(a, b) {
    if (typeof a !== 'string' || typeof b !== 'string') {
        throw new Error('param 'a' and 'b' must be string')
    }
    
    var res = '',
        temp = 0;
    // split the string to an array
    a = a.split('');
    b = b.split('');

    while (a.length || b.length || temp) {
        // the value `true` is number `1`
        // `~~undefined` is number `0`
        temp += ~~a.pop() + ~~b.pop();
        res = (temp % 10) + res;
        temp = temp > 9;
    }
    
    return res.replace(/^0+/, '');
}

// invoke: sumBigNumber('9', '9')
// output: '18'
// invoke: sumBigNumber('123456789123456789', '123456187468716478236487263487623428374628764')
// output: '123456187468716478236487263611080217498085553'