javascript detect undefined

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

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

if (typeof myVariable === 'undefined'){
    //myVariable 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");
}

let foo = undefined
//will return true
typeof foo === 'undefined'

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