Javascript check if undefined

if (typeof myVar !== "undefined") {
    console.log("myVar is DEFINED");
}if (typeof myVariable === 'undefined'){
    //myVariable is undefined
}if(myVar === null) {
	console.log("Element 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...");
}if (typeof myFlag !== "undefined") {
  // comes here whether myFlag is true or false but not defined
}