实现 (5).add(3).minus(2) 功能

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));