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In JavaScript, **a property can be missing entirely**,
or it can **exist but have the value** `undefined`,
and these are two *different situations*!

Let's break down how to tell them apart.

---

## 1. Checking whether a property exists

First, let's recall how to check **whether a property is in the object**:

```javascript
'key' in obj      // true, if the property exists (even if it's undefined)
Object.hasOwn(obj, 'key') // the same thing, just safer
```

But `'key' in obj` doesn't tell you what the property equals,
it only checks that the key **exists in the object**.

---

## 2. Checking that a property **exists but equals** `undefined`

Example situation:

```javascript
const obj = {
  a: undefined
};
```

Now compare it with an object without the property:

```javascript
const obj2 = {};
```

---

## 3. The difference in practice

```javascript
console.log('a' in obj);   // true  (the property exists)
console.log('a' in obj2);  // false (the property does not exist)

console.log(obj.a === undefined);   // true
console.log(obj2.a === undefined);  // true (both return true!)
```

The problem:
a simple check `obj.a === undefined` **does not distinguish**
"the property is missing" from "the property exists, but is undefined".

---

## 4. The solution - combine the checks

To find out for sure that **a property exists and equals** `undefined`, you need:

```javascript
if ('a' in obj && obj.a === undefined) {
  console.log('The property exists, but its value is undefined');
}
```

`'a' in obj` checks **whether the key exists**,
while `obj.a === undefined` checks **the value**.

---

## 5. Example: comparing all the cases

```javascript
const data = {
  a: undefined,
  b: 10
};

console.log('a' in data, data.a); // true, undefined
console.log('b' in data, data.b); // true, 10
console.log('c' in data, data.c); // false, undefined
```

Only `'a' in data` returns `true`,
meaning the key **exists**, even if its value is `undefined`.

---

## Summary

| Check | What it does | Distinguishes key existence? |
|---|---|---|
| `obj.key === undefined` | Checks only the value | No |
| `'key' in obj` | Checks whether the key exists | Yes |
| `'key' in obj && obj.key === undefined` | Checks both conditions | Yes |
| `Object.hasOwn(obj, 'key')` | Modern equivalent of `in` | Yes |

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