javascript check if undefined

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}

1if (typeof myFlag !== "undefined") {
2  // comes here whether myFlag is true or false but not defined
3}

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