how to check variable is undefined in javascript code example

Example 1: javascript check for undefined

if (typeof myVariable === 'undefined'){
    //myVariable is undefined
}

Example 2: javascript if undefined

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

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

} else if(comment === null){

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

}

Example 3: javascript check if boolean is undefined

if (typeof myFlag !== "undefined") {
  // comes here whether myFlag is true or false but not defined
}

Example 4: nodejs check if variable is undefined

if ( typeof query !== 'undefined' && query )
{
  //do stuff if query is defined and not null
}
else
{

}

Example 5: javascript check undefined

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

Example 6: 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))