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## `undefined`

**What it is:**
The value "undefined" (not defined).
Means the **variable exists, but holds nothing**.

**When it appears:**

1. If a variable is declared but **not initialized**:

```javascript
  let user;
  console.log(user); // undefined
  ```
2. If a function **returns nothing**:

```javascript
  function greet() {}
  console.log(greet()); // undefined
  ```
3. If you access a **non-existent object property**:

```javascript
  const obj = {};
  console.log(obj.age); // undefined
  ```

> `undefined` is usually created **automatically** by the JS engine, not manually.

---

## `null`

**What it is:**
The value "empty" or "nothing here", which **the developer assigns explicitly**.

**When to use it:**

1. When you want to **explicitly show** that a value **is absent**:

```javascript
  let user = null; // no user yet
  ```
2. When you need to **clear a variable**:

```javascript
  user = null; // remove the reference to the object
  ```
3. When working with a DB or API, where a field's value can be "empty":

```javascript
  user.email = null; // the user did not provide an email
  ```

> `null` is a **deliberate "nothing"**, while `undefined` is an **accidental "nothing"**.

---

## Comparing `null` and `undefined`

| Property | `undefined` | `null` |
|---|---|---|
| Type (`typeof`) | `"undefined"` | `"object"` *(a spec bug that stuck around historically)* |
| Assigned automatically | Yes | No |
| Assigned manually | Rarely | Often |
| Default value | For uninitialized variables | No |
| Used for "value is absent" | No | Yes |
| Used for "value is not set" | Yes | No |

---

## How to compare them

```javascript
null == undefined   // true  → loose comparison (they are equal by value)
null === undefined  // false → strict comparison (different types)
```

---

## Usage recommendations

- Use `undefined` for:
  - uninitialized variables;
  - missing function arguments;
  - checks for "is there a value" (`if (value === undefined)`).
- Use `null` for:
  - deliberately clearing a value;
  - indicating "value is absent" in program logic or an API;
  - compatibility with JSON and databases.

---

**Example:**

```javascript
let user;              // undefined - the variable is declared, but empty
user = null;           // null - explicitly stated that there is no user
```

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