四舍五入toFoxed方法

四舍五入的方法：

Number.prototype.toFixed = function (n) {
        if (n > 20 || n < 0) {
            throw new RangeError('toFixed() digits argument must be between 0 and 20');
        }
        const number = this;
        if (isNaN(number) || number >= Math.pow(10, 21)) {
            return number.toString();
        }
        if (typeof (n) == 'undefined' || n == 0) {
            return (Math.round(number)).toString();
        }

        let result = number.toString();
        const arr = result.split('.');

        // 整数的情况
        if (arr.length < 2) {
            result += '.';
            for (let i = 0; i < n; i += 1) {
                result += '0';
            }
            return result;
        }

        const integer = arr[0];
        const decimal = arr[1];
        if (decimal.length == n) {
            return result;
        }
        if (decimal.length < n) {
            for (let i = 0; i < n - decimal.length; i += 1) {
                result += '0';
            }
            return result;
        }
        result = integer + '.' + decimal.substr(0, n);
        const last = decimal.substr(n, 1);

        // 四舍五入，转换为整数再处理，避免浮点数精度的损失
        if (parseInt(last, 10) >= 5) {
            const x = Math.pow(10, n);
            result = (Math.round((parseFloat(result) * x)) + 1) / x;
            result = result.toFixed(n);
        }

        return result;
    };

核心方法：

function tofixed(num,n){
    let result = num.toString();
    const arr = result.split('.');
    const integer = arr[0];
    const decimal = arr[1];
    if (decimal.length == n) { //小数位等于要求长度，无需处理
        return result;
    }
    if (decimal.length < n) {
        result = integer + '.' + decimal.padEnd(2,'0');//小数位不够要求长度，补位
        return result;
    }

    result = integer + '.' + decimal.substr(0, n);
    const last = decimal.substr(n, 1);

    // 四舍五入，转换为整数再处理，避免浮点数精度的损失
    if (parseInt(last, 10) >= 5) {
        const x = Math.pow(10, n); //10的n次方
        result = (Math.round((parseFloat(result) * x)) + 1) / x;
        result = result.toFixed(n);
    }

    return result;
}

let handleNum = tofixed(10.234,2);
console.log(handleNum); //10.23
let handleNum2 = tofixed(10.235,2);
console.log(handleNum2); //10.24
let handleNum3 = tofixed(10.5,2);
console.log(handleNum3); //10.50