大整数相乘：

function multi(a,b){
    var str1,str2,len1,len2,maxlen,result = [];
    str1 = a.split("").reverse();
    str2 = b.split("").reverse();
    len1 = str1.length;
    len2 = str2.length;

    for(var i = 0;i < len1;i++){
        for(var j = 0;j < len2;j++){
            result[i + j] = 0;
        }
    }
    for(var i = 0;i < len1;i++){
        for(var j = 0;j < len2;j++){
            result[i + j] += parseInt(str1[i]) * parseInt(str2[j]);
        }
    }
    var n = result.length;
    for(var k = 0;k < n;k++){
        var temp = result[k];
        if(temp >= 10)
        {
            result[k] = temp % 10;
            if(result[k + 1] === undefined){
                result[k + 1] = 0;
            }
            result[k + 1] +=  Math.floor(temp / 10);
        }
    }
    return result.reverse().join("");
}

例子：

multi('5158','878361')
"4530586038"

字符串乘以n个0：

function StrPointAdd(str,s){
    var resData = '';
    if(str&&str!==''){
        var strArr = str.split('.');
        if(strArr.length===1){
        resData = strArr[0];
        for(var i=0;i<s;i++){
            resData = resData + '0';
        }
        }else if(strArr.length===2){
        var l = strArr[1].length;
        var cha = l-s;
        if(cha>=0){
            resData = strArr[1];
            resData = resData.substring(0,l-cha)+'.'+resData.substring(l-cha);
            resData = strArr[0] + resData;
        }else{
            resData = strArr[1];
            for(var i=0;i<-cha;i++){
            resData = resData + '0';
            }
            resData = strArr[0] + resData;
        }
        }
        //去除多余的0
        var res_ = resData;
        if(res_.indexOf('.')!==-1){
        for(var i=0;i<res_.length;i++){
            if(res_[res_.length-1-i]=='0'){
            resData = res_.slice(0,res_.length-1-i);
            }else if(res_[res_.length-1-i]=='.'){
            resData = res_.slice(0,res_.length-1-i);
            break
            }else{
            break
            }
        }
        }
        res_ = resData;
        for(var i=0;i<res_.length;i++){
        if(res_[i]=='0'){
            resData = res_.slice(i+1);
        }else if(res_[i]=='.'){
            resData = '0'+res_.slice(i);
            break;
        }else{
            break;
        }
        }
    }
    return resData;
}

例子：

StrPointAdd('123456789',5)
"12345678900000"

StrPointAdd('0.123456789',5)
"12345.6789"

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