is undefined javascript

if (typeof myVar !== "undefined") {
    console.log("myVar is DEFINED");
}//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 (typeof myFlag !== "undefined") {
  // comes here whether myFlag is true or false but not defined
}if(undefinedElement === null) {
	console.log("Element is undefined");
}