Javascript Data Type check if a defined variable has a value using equality operators

// myvar is undefined 
var myvar = undefined; 
if ( myvar === undefined) {
   console.log("myvar is undefined"); 
} 
if ( myvar === null) {
   console.log("myvar is null"); 
} 

// myvar is null 
var myvar = null; 
if ( myvar === undefined) {
   console.log("myvar is undefined"); 
} 
if ( myvar === null) {
   console.log("myvar is null"); 
} 

var myvar = null; 

// check if myvar is null using the bang (!) operator 
if ( !myvar ) { //from  w w w . j  a  v  a  2  s  .co  m
   console.log("The variable myvar is null or undefined"); 
} else { 
   console.log("The variable myvar is not null or undefined"); 
}