is undefined javascript

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

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");
}

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

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");
}

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

OAS3