This error occurs when we try to access a property or a method of an object
that is undefined or null. For example:
const person = {
name: "Kayhan",
age: 25
};
console.log(person.job.title);
The code above will throw a TypeError because person.job is undefined and we
cannot read its title property. To fix this error, we need to check if
person.job exists before accessing its title property:
const person = {
name: "Kayhan",
age: 25
};
if (person.job) {
console.log(person.job.title);
}
To avoid this error, we should always check if an object or its property
exists before accessing it.

// We can fix the error by adding an optional chaining operator (?.) on the variable before accessing a property.
// If the variable is undefined or null, the operator will return undefined immediately and prevent the property access.
const auth = undefined;
console.log(auth); // undefined
// ❌ TypeError: Cannot read properties of undefined (reading 'user')
console.log(auth.user.name);
// ✅ No error
console.log(auth?.user?.name); // undefined

/** * Depending on your scenario, doing any one of the following might resolve the error: * * 1. Add an undefined check on the variable before you access it. * 2. Get the property/method from a replacement for the undefined variable. * 3. Use a fallback result instead of accessing the property. * 4. Check your code to find out why the variable is undefined. */
const auth = undefined;
console.log(auth); // undefined
// ❌ TypeError: Cannot read properties of undefined (reading 'user')
console.log(auth.user.name);
const auth = undefined;
console.log(auth); // undefined
// ✅ No error
console.log(auth?.user?.name); // undefined
const auth = undefined;
console.log(auth); // undefined
// ✅ No error
console.log(auth?.['user']?.['name']); // undefined
const arr = undefined;
console.log(arr?.[0]); // undefined
// Array containing an object
console.log(arr?.[2]?.prop); // undefined