Answers for "js function undefined"

js if not undefined

if (typeof myVar !== "undefined") {
    console.log("myVar is DEFINED");
}

javascript if undefined

if(typeof comment === 'undefined') {

  alert('Variable "comment" is undefined.');

} else if(comment === null){

  alert('Variable "comment" is null.');

}

js return undefined on ReferenceError

window.console && console.log(new Date());