基本用法：typeof是引擎提供的内置方法，执行结果返回字符串，且均为小写，包括number,string,boolean,undefined,object,function,symbol
举例说明： typeof ""; //string
typeof 1; //number
typeof false; //boolean
typeof undefined; //undefined
**typeof function(){}; //function **
typeof {}; //object
typeof Symbol(); //symbol
typeof null; //object
typeof []; //object
typeof new Date(); //object
typeof new RegExp(); //object
