typeof checks the data type of a variable

typeof returns 6 data types:

• object
• undefined
• string
• number
• boolean
• function

　　var a;

　　var b=1;

　　var c = 'nihao';

　　var d =

var a=null;
var b;
var c="nihao";
var d=12;
var e=true;

　　

console.log(typeof a);     //object
console.log(typeof b);     //undefined

console.log(b1);           //undefined
console.log(typeof c);     //string
console.log(typeof d);     //number
console.log(typeof e);     //boolean

console.log(function(){});  //function

Note: When the variable is null, the return type is object, which is a continuation of the javascript error; and when the variable does not exist or the variable is not assigned a value, the return type is object;