1if (typeof myVar !== "undefined") {
2    console.log("myVar is DEFINED");
3}

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  }

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