javascript is undefined

if (typeof myVariable === 'undefined'){
    //myVariable is undefined
}var x;
if (typeof x === 'undefined') {
   // these statements execute
}if (typeof something === "undefined") {
    alert("something 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");
}function divisors(integer) {
    let  i, flag = true; 
       let arr=[];       
           
            
            for(i = 2; i <= integer - 1; i++) {
                if (integer % i == 0) { 
                    flag = false; 
                     arr.push(i)
                } 
            }
            if (flag == true) 
              console.log(integer + " is prime"); 
            else
                console.log(arr)
}
console.log(divisors(15))