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Short answer (EN)

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Answer (EN)

## Short answer

If you access a **nonexistent property of an object**,
no error occurs,
and the result is `undefined`.

---

## Example

```javascript
const user = { name: 'Oleh' };

console.log(user.name);     // "Oleh"
console.log(user.age);      // undefined
console.log(user.address);  // undefined
```

> JavaScript does not throw an error, it just returns `undefined`.

---

## Why this happens

When JS accesses a property, it:

1. Checks whether it exists **on the object**.
2. If it **exists** → returns its value.
3. If it **does not** → returns `undefined`.

That is:

```javascript
user.hasOwnProperty('age'); // false
user.age; // undefined
```

---

## But important: an error appears if you go further down the chain

```javascript
console.log(user.address.city); // TypeError
```

> Why?
> `user.address` is `undefined`,
> and you **cannot request properties** (`city`) on `undefined`.

---

## How to avoid the error when accessing nested properties

### Method 1. Manual checks

```javascript
if (user.address && user.address.city) {
  console.log(user.address.city);
}
```

---

### Method 2. The optional chaining operator `?.` (the modern approach)

```javascript
console.log(user.address?.city); // undefined (but no error!)
```

> If the left side of `?.` is `null` or `undefined`,
> JS **stops evaluation** and returns `undefined`.

---

### Method 3. Use a default value via `??`

```javascript
console.log(user.address?.city ?? 'Not specified');
// "Not specified"
```

---

## Example with a dynamic key

```javascript
const key = 'email';
console.log(user[key]); // undefined (if the property does not exist)
```

> Even with dynamic access via `[]` the behavior is the same, just `undefined`.

---

## Difference from other cases

| Situation | Result | Explanation |
|---|---|---|
| `user.age` | `undefined` | The property is missing |
| `user.age.city` | `TypeError` | `undefined` is not an object |
| `'age' in user` | `false` | Checks whether the property exists |
| `user.hasOwnProperty('age')` | `false` | Checks only own properties |
| `user.name ?? 'no'` | `'Oleh'` | Returns the value |
| `user.age ?? 'no'` | `'no'` | Default value |

---

## Summary

| Behavior | What it does |
|---|---|
| `obj.missing` | returns `undefined` |
| No error | even if the property is missing |
| An error appears | when trying to go deeper (`undefined.city`) |
| Solution | use `?.` or `??` |

---

### Example to reinforce this

```javascript
const user = { name: 'Oleh' };

console.log(user.age);           // undefined
console.log(user.age?.value);    // undefined (no error)
console.log(user.age.value);     // TypeError
console.log(user.age ?? 'no');   // "no"
```

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