用 Javascr ipt 编写一个简单的计算器。实现一个标准型计算器的基本功能；实现加，减，乘，除，取余，开平方，求倒数等基本运算；

支持正负号，小数点输入；实现Backspace, CE, C键的功能；-A simple calculator written in Javascr ipt. Achieve the basic functions of a standard calculator　addition, subtraction, multiplication, division, modulo, square root, seeking reciprocal basic operations

Sign, decimal point input　Backspace, CE, C key functions