SOURCE

let Float_Dolphin = function () {

    /*
     * 判断obj是否为一个整数
     */
    function isInteger(obj) {
        return Math.floor(obj) === obj
    }

    /*
     * 将一个浮点数转成整数，返回整数和倍数。如 3.14 >> 314，倍数是 100
     * @param floatNum {number} 小数
     * @return {object}
     *   {times:100, num: 314}
     */
    function toInteger(floatNum) {
        let ret = {times: 1, num: 0};
        if (isInteger(floatNum)) {
            ret.num = floatNum;
            return ret
        }
        let strfi = floatNum + '';
        let dotPos = strfi.indexOf('.');
        let len = strfi.substr(dotPos + 1).length;
        let times = Math.pow(10, len);
        let intNum = parseInt(floatNum * times + 0.5, 10);
        ret.times = times;
        ret.num = intNum;
        return ret
    }

    /*
     * 核心方法，实现加减乘除运算，确保不丢失精度
     * 思路：把小数放大为整数（乘），进行算术运算，再缩小为小数（除）
     *
     * @param a {number} 运算数1
     * @param b {number} 运算数2
     * @param op {string} 运算类型，有加减乘除（add/subtract/multiply/divide）
     *
     */
    function operation(a, b, op) {
        let o1 = toInteger(a);
        let o2 = toInteger(b);
        let n1 = o1.num;
        let n2 = o2.num;
        let t1 = o1.times;
        let t2 = o2.times;
        let max = t1 > t2 ? t1 : t2;
        let result = null;
        switch (op) {
            case 'add':
                if (t1 === t2) { // 两个小数位数相同
                    result = n1 + n2
                } else if (t1 > t2) { // o1 小数位 大于 o2
                    result = n1 + n2 * (t1 / t2)
                } else { // o1 小数位 小于 o2
                    result = n1 * (t2 / t1) + n2
                }
                return result / max;
            case 'subtract':
                if (t1 === t2) {
                    result = n1 - n2
                } else if (t1 > t2) {
                    result = n1 - n2 * (t1 / t2)
                } else {
                    result = n1 * (t2 / t1) - n2
                }
                return result / max;
            case 'multiply':
                result = (n1 * n2) / (t1 * t2);
                return result;
            case 'divide':
                result = (n1 / n2) * (t2 / t1);
                return result
        }
    }

    // 加减乘除的四个接口
    function add(a, b) {//加
        return operation(a, b, 'add')
    }

    function subtract(a, b) {//减
        return operation(a, b, 'subtract')
    }

    function multiply(a, b) {//乘
        return operation(a, b, 'multiply')
    }

    function divide(a, b) {//除
        return operation(a, b, 'divide')
    }

    // exports
    return {
        add: add,
        subtract: subtract,
        multiply: multiply,
        divide: divide
    }
}();



 console.log(Float_Dolphin.add(0.1, 0.2));//0.3
 console.log(Float_Dolphin.subtract(1.0, 0.9));//0.1
 console.log(Float_Dolphin.multiply(19.9, 100));//1990
 console.log(Float_Dolphin.divide(6.6, 0.2));//33