Answers for "{} null or undefined"

js is undefined or null

var myVar=null;

if(myVar === null){
    //I am null;
}

if (typeof myVar === 'undefined'){
    //myVar is undefined
}

null vs undefined

//loosely equal (compare values between two variables)
null == undefined // true  ( null => 0 , undefined => NAN)

//strictly not equal (compare both type and value) 
null === undefined // false  (typeof null => object , typeof undefined => undefined)
null !== undefined // true   (typeof null => object , typeof undefined => undefined)