A JavaScript error: what does undefined mean?

undefined is not always an error. If a variable was created but never given a value, its value is undefined.

let ad;
console.log(ad); // undefined
ad = "Murad";
console.log(ad); // Murad

A missing array index and a missing object property also return undefined. That says the data is not there. It does not stop the program by itself.

let meyveler = ["Alma", "Armud", "Banan"];
console.log(meyveler[5]); // undefined
let telebe = { ad: "Aysel", yas: 20 };
console.log(telebe.soyad); // undefined

null is an empty value the programmer set on purpose. undefined means a value was never given. Reading a property from undefined throws a TypeError. Check first.

let secilmis = null;
let telebe;
if (telebe !== undefined) {
  console.log(telebe.ad);
} else {
  console.log("Tələbə məlumatı yoxdur.");
}

Is undefined an error? By itself, no. A value was not given, or the data you asked for is missing.

How is it different from null? null was left empty on purpose. undefined was never set.

When does a TypeError happen? When you read a property from undefined with a dot. For example telebe.ad while telebe is undefined.

How do I check? if (ad === undefined). If there is a value, ad !== undefined.