js运算精度问题

//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) {}
        with(Math) {
            r1 = Number(this.toString().replace(".", ""))
            r2 = Number(arg.toString().replace(".", ""))
            return (r1 / r2) * pow(10, t2 - t1);
        }
    }