Проверка JavaScript для неопределенного
if (typeof myVariable === 'undefined'){
//myVariable is undefined
}
Grepper
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.');
}Your question goes here...What is undefinedThe undefined property indicates that a variable has not been assigned a value,
or not declared at all.let name;
console.log(name); // undefined