null

null

// foo does not exist. It is not defined and has never been initialized:
foo;
"ReferenceError: foo is not defined"

// foo is known to exist now but it has no type or value:
var foo = null; 
foo;
"null"

typeof null          // "object" (not "null" for legacy reasons)
typeof undefined     // "undefined"
null === undefined   // false
null  == undefined   // true
null === null        // true
null == null         // true
!null                // true
isNaN(1 + null)      // false
isNaN(1 + undefined) // true