Number.prototype.add = function (number) {

    if (typeof number !== 'number') {

        throw new Error('请输入数字～');

    }

    return this.valueOf() + number;

};

Number.prototype.minus = function (number) {

    if (typeof number !== 'number') {

        throw new Error('请输入数字～');

    }

    return this.valueOf() - number;

};

console.log((5).add(3).minus(2));