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## What Happens if There Is No Constructor

If you don't define a `constructor()`,
JavaScript **automatically creates one by default**.

### Example:

```javascript
class User {
  greet() {
    console.log("Hi!");
  }
}

const u = new User();
u.greet(); // Hi!
```

Under the hood, the engine creates **a constructor like this**:

```javascript
class User {
  constructor(...args) {
    super(...args); // if there is a parent class
  }
}
```

---

## If a Class **Does Not Inherit** (No `extends`)

Then the standard "empty" constructor simply does nothing:

```javascript
class User {}

// equivalent to:
class User {
  constructor() {}
}
```

---

## If a Class **Inherits** From Another (`extends` Is Present)

Then a **constructor is required only if you want to add logic and use** `this`.
If there isn't one, the engine adds a constructor itself that calls `super()`.

```javascript
class Animal {
  constructor(name) {
    this.name = name;
  }
}

class Dog extends Animal {}

// Equivalent to:
class Dog extends Animal {
  constructor(...args) {
    super(...args);
  }
}

const dog = new Dog("Rex");
console.log(dog.name); // Rex
```

---

## Important: If You Add a Constructor in the Subclass Yourself

You **must call** `super()` before using `this`, otherwise you get an error.

```javascript
class Animal {
  constructor(name) {
    this.name = name;
  }
}

class Dog extends Animal {
  constructor(name, breed) {
    // super() must come first!
    super(name);
    this.breed = breed;
  }
}
```

If you don't call it:

```javascript
constructor(name, breed) {
  this.breed = breed; // ReferenceError: Must call super constructor before accessing 'this'
}
```

---

## Summary

| Situation | Do You Need to Write `constructor()`? |
| --- | --- |
| Class without inheritance | Not needed |
| Class inherits another one but adds no new behavior | Not needed |
| Class inherits and adds its own fields | Needed (and must call `super()`) |
| Logic must run when the instance is created (e.g., initialization, validation) | Needed |

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