JavaScript实现大整数减法

function subString(a,b) {
    //将字符串a和b补全成同等长度
    while (a.length < b.length){
        a = '0' + a;
    }
    while (b.length < a.length){
        b = '0' + b;
    }
    //res保存结果，c用来标识有无借位的情况
    var res='', c=0;
    a = a.split('');
    b = b.split('');
    while (a.length) {
        var num1 = ~~a.pop();
        var num2 = ~~b.pop();
        if (num1 >= num2){
            c = num1 - num2 - c;
            res = c + res;
            c = false;
        }else {
            c = num1 + 10 - num2 - c;
            res = c + res;
            c = true
        }
        //判断最高位有无借位，若有借位，则说明结果为负数
        if (a.length === 0 && c){
            res = '-' + res
        }

    }
    res = res.replace(/^0+/,'');
    //判断最后的结果是否为0
    if (res === ''){
        res = 0;
    }
    return res;
}