js check if undefined

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

if(myVar === null) {
	console.log("Element is undefined");
}

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

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

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

} else if(comment === null){

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

}

let foo = undefined
//will return true
typeof foo === 'undefined'

if(undefinedElement === null) {
	console.log("Element is undefined");
}

javascript check number against undefined

OAS3