Introduction to the difference between undefined and null in js_Basic knowledge

When a variable is declared but not assigned a value and a variable is not declared, it is undefined type data
JS variables must also be declared before use
If an undefined variable x is used, an error will be reported.
How to determine whether a variable is available:

The first method:
if(typeof x=='undefined')
var n=100;
n=new Date();
n= null;
null represents a clear known value, which is an empty object

Second method:

if(typeof(y)!='undefined' && v!=null)//You can also determine whether a variable is available

Third method:

var v;

if(v)

{//v will be converted to Boolean type and then judged. 

alert('available');

        }

else{

alert('unavailable');

         }

We rarely use the first and second methods, and usually use the third.                             

s, undefined and null are equal if == is used, and unequal if === is used.