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

1if ( typeof query !== 'undefined' && query )
2{
3  //do stuff if query is defined and not null
4}
5else
6{
7
8}
9

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