typeof

语法：typeof(表达式)和typeof xxx

值：undefined object boolean number string function symbol bigint

栗子:

typeof NaN  //number
typeof Infinity //number
typeof Math.LN2 //number

typeof typeof(1) //string
typeof 42n //bigint
typeof Boolean(1)  //boolean
typeof !!1 // boolean

typeof Symbol() //symbol
typeof Symbol.iterator //symbol

typeof undefined // 'undefined';
typeof abc //'undefined';

typeof {} //object
typeof [] //object
typeof /d/ //object
typeod null //object

typeof new Boolean(true)  //object
typeof new Number(1) //object
typeof new String('abc') //object

typeof function() {} //function
typeof class C {} //function
typeof Math.sin //function

typeof Math.sin() //number