js is undefined

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

if(typeof id === 'undefined') {
    console.log("id is undefined...");
}if (typeof myFlag !== "undefined") {
  // comes here whether myFlag is true or false but not 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");
}