Edit in JSFiddle

function QiuSum(n){
    var s = n.toString();
    var sum = 0;
    for(var i=0; i<s.length; i++){
        sum += parseInt(s[i]);
    }
    return sum;
}

function QiuM(n){
    var m = n;
    var length4N = n.toString().length;
    var sum4N = QiuSum(n)
    while(true){
        m++;
        if(m.toString().length > length4N){
            return 0;
        }
        
        if(QiuSum(m) == QiuSum(n)){
            return m;
        }
    }
}

document.getElementById("inputCalc").onclick = function(){
    var inputN = document.getElementById("inputN");
    var inputM = document.getElementById("inputM");
    var n = parseInt(inputN.value);
    var m = QiuM(n);
    inputM.value = m;
}

<b>给定一个整数N，求编码函数f，f满足M=f(N)，M可以不存在，且M和n的关系满足：
</b>
<ul>
    <li>M的位数和N相同</li>
    <li>M的各个位之和与N的各个位之和相同</li>
    <li>M>N</li>
    <li>M的各M是满足上述条件中，值最小的一个解<li>
</ul>

<br />
N=<input id="inputN" />，M=<input id="inputM" />,<input id="inputCalc" type="button" value="计算" />