// 加法 =====================
0.1 + 0.2 = 0.30000000000000004
0.7 + 0.1 = 0.7999999999999999
0.2 + 0.4 = 0.6000000000000001

// 減法 =====================
1.5 - 1.2 = 0.30000000000000004
0.3 - 0.2 = 0.09999999999999998
 
// 乘法 =====================
19.9 * 100 = 1989.9999999999998
0.8 * 3 = 2.4000000000000004
35.41 * 100 = 3540.9999999999995

// 除法 =====================
0.3 / 0.1 = 2.9999999999999996
0.69 / 10 = 0.06899999999999999

1.35.toFixed(1) // 1.4 正確
1.335.toFixed(2) // 1.33  錯(cuò)誤
1.3335.toFixed(3) // 1.333 錯(cuò)誤
1.33335.toFixed(4) // 1.3334 正確
1.333335.toFixed(5)  // 1.33333 錯(cuò)誤
1.3333335.toFixed(6) // 1.333333 錯(cuò)誤

0.1 -> 0.0001 1001 1001 1001...(1100循環(huán))
0.2 -> 0.0011 0011 0011 0011...(0011循環(huán))

0.0100110011001100110011001100110011001100110011001100 

// toFixed兼容方法
Number.prototype.toFixed = function(len){
    if(len>20 || len<0){
        throw new RangeError('toFixed() digits argument must be between 0 and 20');
    }
    // .123轉(zhuǎn)為0.123
    var number = Number(this);
    if (isNaN(number) || number >= Math.pow(10, 21)) {
        return number.toString();
    }
    if (typeof (len) == 'undefined' || len == 0) {
        return (Math.round(number)).toString();
    }
    var result = number.toString(),
        numberArr = result.split('.');

    if(numberArr.length<2){
        //整數(shù)的情況
        return padNum(result);
    }
    var intNum = numberArr[0], //整數(shù)部分
        deciNum = numberArr[1],//小數(shù)部分
        lastNum = deciNum.substr(len, 1);//最后一個(gè)數(shù)字
    
    if(deciNum.length == len){
        //需要截取的長(zhǎng)度等于當(dāng)前長(zhǎng)度
        return result;
    }
    if(deciNum.length < len){
        //需要截取的長(zhǎng)度大于當(dāng)前長(zhǎng)度 1.3.toFixed(2)
        return padNum(result)
    }
    //需要截取的長(zhǎng)度小于當(dāng)前長(zhǎng)度，需要判斷最后一位數(shù)字
    result = intNum + '.' + deciNum.substr(0, len);
    if(parseInt(lastNum, 10)>=5){
        //最后一位數(shù)字大于5，要進(jìn)位
        var times = Math.pow(10, len); //需要放大的倍數(shù)
        var changedInt = Number(result.replace('.',''));//截取后轉(zhuǎn)為整數(shù)
        changedInt++;//整數(shù)進(jìn)位
        changedInt /= times;//整數(shù)轉(zhuǎn)為小數(shù)，注：有可能還是整數(shù)
        result = padNum(changedInt+'');
    }
    return result;
    //對(duì)數(shù)字末尾加0
    function padNum(num){
        var dotPos = num.indexOf('.');
        if(dotPos === -1){
            //整數(shù)的情況
            num += '.';
            for(var i = 0;i<len;i++){
                num += '0';
            }
            return num;
        } else {
            //小數(shù)的情況
            var need = len - (num.length - dotPos - 1);
            for(var j = 0;j<need;j++){
                num += '0';
            }
            return num;
        }
    }
}

0.1 + 0.2 == 0.3 //false
(0.1*10 + 0.2*10)/10 == 0.3 //true

35.41 * 100 = 3540.9999999999995

 /*** method **
 *  add / subtract / multiply /divide
 * floatObj.add(0.1, 0.2) >> 0.3
 * floatObj.multiply(19.9, 100) >> 1990
 *
 */
var floatObj = function() {
    /*
     * 判斷obj是否為一個(gè)整數(shù)
     */
    function isInteger(obj) {
        return Math.floor(obj) === obj
    }

    /*
     * 將一個(gè)浮點(diǎn)數(shù)轉(zhuǎn)成整數(shù)，返回整數(shù)和倍數(shù)。如 3.14 >> 314，倍數(shù)是 100
     * @param floatNum {number} 小數(shù)
     * @return {object}
     *   {times:100, num: 314}
     */
    function toInteger(floatNum) {
        var ret = {times: 1, num: 0}
        if (isInteger(floatNum)) {
            ret.num = floatNum
            return ret
        }
        var strfi  = floatNum + ''
        var dotPos = strfi.indexOf('.')
        var len    = strfi.substr(dotPos+1).length
        var times  = Math.pow(10, len)
        var intNum = Number(floatNum.toString().replace('.',''))
        ret.times  = times
        ret.num    = intNum
        return ret
    }

    /*
     * 核心方法，實(shí)現(xiàn)加減乘除運(yùn)算，確保不丟失精度
     * 思路：把小數(shù)放大為整數(shù)（乘），進(jìn)行算術(shù)運(yùn)算，再縮小為小數(shù)（除）
     *
     * @param a {number} 運(yùn)算數(shù)1
     * @param b {number} 運(yùn)算數(shù)2
     * @param digits {number} 精度，保留的小數(shù)點(diǎn)數(shù)，比如 2, 即保留為兩位小數(shù)
     * @param op {string} 運(yùn)算類型，有加減乘除（add/subtract/multiply/divide）
     *
     */
    function operation(a, b, digits, op) {
        var o1 = toInteger(a)
        var o2 = toInteger(b)
        var n1 = o1.num
        var n2 = o2.num
        var t1 = o1.times
        var t2 = o2.times
        var max = t1 > t2 ? t1 : t2
        var result = null
        switch (op) {
            case 'add':
                if (t1 === t2) { // 兩個(gè)小數(shù)位數(shù)相同
                    result = n1 + n2
                } else if (t1 > t2) { // o1 小數(shù)位 大于 o2
                    result = n1 + n2 * (t1 / t2)
                } else { // o1 小數(shù)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
        }
    }

    // 加減乘除的四個(gè)接口
    function add(a, b, digits) {
        return operation(a, b, digits, 'add')
    }
    function subtract(a, b, digits) {
        return operation(a, b, digits, 'subtract')
    }
    function multiply(a, b, digits) {
        return operation(a, b, digits, 'multiply')
    }
    function divide(a, b, digits) {
        return operation(a, b, digits, 'divide')
    }

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