javascript if 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.');

}let foo = undefined
//will return true
typeof foo === '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");
}var x; //variable to change
var y; //higher priority variable
var z = 12; //lower priority variable
x = !(y === undefined) ? y : z;
console.log(x); //12
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