is undefined javascript

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

}if (typeof something === "undefined") {
    alert("something is undefined");
}let id;

if(typeof id === 'undefined') {
    console.log("id is undefined...");
}//detecting undefined object property
if (typeof person.last_name === "undefined") {
    console.log("person last_name is undefined");
}

//detecting undefined variable 
if (typeof myVar === "undefined") {
    console.log("myVar is undefined");
}