1if (typeof myVariable === 'undefined'){
2    //myVariable is undefined
3}

1if(typeof comment === 'undefined') {
2
3  alert('Variable "comment" is undefined.');
4
5} else if(comment === null){
6
7  alert('Variable "comment" is null.');
8
9}

1let id;
2
3if(typeof id === 'undefined') {
4    console.log("id is undefined...");
5}

1 if (t === undefined) {
2    return 'Undefined value!';
3  }

1let foo = undefined
2//will return true
3typeof foo === 'undefined'

1if(undefinedElement === null) {
2	console.log("Element is undefined");
3}