undefined javascript check

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

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 id;

if(typeof id === 'undefined') {
    console.log("id 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");
}

OAS3

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))