js浮点数加减乘除精度不准确

做个记录，以备不时之需
 //加法
    Number.prototype.add = function(arg){
        var r1,r2,m;
        try{r1=this.toString().split(".")[1].length}catch(e){r1=0}
        try{r2=arg.toString().split(".")[1].length}catch(e){r2=0}
        m=Math.pow(10,Math.max(r1,r2))
        return (this*m+arg*m)/m
    }
//减法
Number.prototype.sub = function (arg){
    return this.add(-arg);
}   

//乘法
Number.prototype.mul = function (arg)
{
    var m=0,s1=this.toString(),s2=arg.toString();
    try{m+=s1.split(".")[1].length}catch(e){}
    try{m+=s2.split(".")[1].length}catch(e){}
    return Number(s1.replace(".",""))*Number(s2.replace(".",""))/Math.pow(10,m)
}   

//除法
Number.prototype.div = function (arg){
    var t1=0,t2=0,r1,r2;
    try{t1=this.toString().split(".")[1].length}catch(e){}
    try{t2=arg.toString().split(".")[1].length}catch(e){}
    r1=Number(this.toString().replace(".",""))
    r2=Number(arg.toString().replace(".",""))
    return (r1/r2)*Math.pow(10,t2-t1);
}