how to check a variable\ in javascript is undefined code example

Example 1: javascript if undefined

if(typeof comment === 'undefined') {

  alert('Variable "comment" is undefined.');

} else if(comment === null){

  alert('Variable "comment" is null.');

}

Example 2: undefined value check in javascript

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))