var a = 2.255;

var b = a.toFixed(2);

console.log(b);

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;

};