(5).add(3).minus(2) 即5+3-2=6

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));