js解决数字运算丢失精度，保留小数位

/**
* 左补齐字符串
* @param  {Number} nLen    要补齐的长度
* @param  {String} ch     要补齐的字符
* @return {String}        补齐后的字符串
*/
var padLeft = function (str, nLen, ch) {
    var len = 0,
        s = str ? str : "";
    // 默认要补齐0
    ch = ch ? ch : '0';
    len = s.length;
    while (len < nLen) {
        s = ch + s;
        len++;
    }
    return s;
}


/**
 * 右补齐字符串
 * @param  {Number} nLen    要补齐的长度
 * @param  {String} ch         要补齐的字符
 * @return {String}            补齐后的字符串
 */
var padRight = function (str, nLen, ch) {
    var len = 0,
        s = str ? str : "";
    // 默认要补齐0
    ch = ch ? ch : '0';
    len = s.length;
    while (len < nLen) {
        s = s + ch;
        len++;
    }
    return s;
}


/**
 * 左移小数点的位置（用于数学计算，相当于除以Math.pow(10, scale)
 * @param  {Number} str 要移动的字符串
 * @param  {Number} scale     要移位的刻度
 * @return {String}         返回移位后的数字字符串
 */
var movePointLeft = function (str, scale) {
    var s, s1, s2, ch, ps, sign;
    ch = '.';
    sign = '';
    s = str ? str : "";
    if (scale <= 0) return s;
    ps = s.split('.');
    s1 = ps[0] ? ps[0] : "";
    s2 = ps[1] ? ps[1] : "";
    if (s1.slice(0, 1) == "-") {
        // 负数
        s1 = s1.slice(1);
        sign = '-';
    }
    if (s1.length <= scale) {
        ch = "0.";
        s1 = this.padLeft(s1, scale);
    }
    return sign + s1.slice(0, -scale) + ch + s1.slice(-scale) + s2;
}


/**
 * 右移小数点位置（用于数学计算，相当于乘以Math.pow(10, scale)
 * @param  {Number} str 要移动的字符串
 * @param  {Number} scale     要移位的刻度
 * @return {String}         返回移位后的数字字符串
 */
var movePointRight = function (str, scale) {
    var s, s1, s2, ch, ps;
    ch = '.';
    s = str ? str : "";
    if (scale <= 0) return s;
    ps = s.split('.');
    s1 = ps[0] ? ps[0] : "";
    s2 = ps[1] ? ps[1] : "";
    if (s2.length <= scale) {
        ch = '';
        s2 = this.padRight(s2, scale, ch);
    }
    return s1 + s2.slice(0, scale) + ch + s2.slice(scale, s2.length);
}


/**
 * 移动小数点位置（用于数学计算，相当于（乘以／除以）Math.pow(10, scale)
 *  @param  {Number} str 要移动的字符串
 * @param  {Number} scale         要移位的刻度（正数表示向右移动；负数表示向左移动；0返回原值）
 * @return {String}             返回移位后的数字字符串
 */
var movePoint = function (str, scale) {
    if (scale >= 0) {
        return this.movePointRight(str, scale);
    } else {
        return this.movePointLeft(str, -scale);
    }
}

// 提高数字的易读性，在数字每隔3位处增加逗号
var prettyNumeric = function (num, separator) {
    if (isNaN(num)) return num.toString();
    var s = num.toString().split('.'),
        isNegative = num < 0 ? true : false,
        s1 = isNegative ? s[0].replace('-', '') : s[0],
        s2 = s[1] || '',
        l = s1.length,
        r = '';
    separator = !separator ? ',' : separator;
    if (l > 3) {
        var l1 = parseInt(l / 3),
            idx = l % 3;
        r = idx == 0 ? '' : s1.slice(0, idx) + separator;
        for (var i = 0; i < l1; i++) {
            r += s1.slice(idx + (i * 3), (idx + (i + 1) * 3)) + separator;
        }
        r = (isNegative ? '-' : '') + r.slice(0, -1) + (s2.length > 0 ? ('.' + s2) : '');
    } else {
        r = num;
    }
    return r;
};