Javascript Data Type check difference between null and undefined

var a; // w  ww  . j a va 2s . c  om
var b = null; 

if ( typeof a === 'undefined' ) {
          console.log( "Variable a is undefined" ); 
} 

if ( a === null ) {
          console.log( "Variable a is null" ); 
} 

if ( typeof b === 'undefined' ) {
          console.log( "Variable b is undefined" ); 
} 

if ( b === null ) {
          console.log( "Variable b is null" ); 
}