JavaScript data types

JS data types have undefined,boolean,number,string,object, such as 5, the first 4 are the original type, the 5th is the reference type.

Something special to be aware of. Null undefined NAN

(1) Undefined and null are equal, (2) Nan is not equal to any value, nor is it equal to itself.

var a1; var a2 = true;  var a3 = 1; var a4 =  "Hello";  var a5 = new object ( );  var a6 = null; var a7 = nan; var a8 = undefined ;  alert (typeof a);  //displays "Undefined"  alert (TYPEOF A1);  //displays "Undefined"  alert ( TYPEOF A2);  //displays "Boolean"  alert (TYPEOF A3);  //displays "number"  alert (TYPEOF A4);  //displays "string"  alert (TYPEOF A5);  //displays "Object"  alert (TYPEOF A6);  //displays "Object"  alert (TYPEOF A7);  //displays "number"  alert (TYPEOF A8);  //displays "Undefined" 

JavaScript Undefined Properties

Definition and Usage  
The undefined property is used to store JavaScript undefined values.

JavaScript data types