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## Short answer

**Yes, you can!**
You can just assign the object a new property whose value is a function.

---

## Example

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

// add a method after creation
user.sayHi = function() {
  console.log(`Hello, ${this.name}!`);
};

user.sayHi(); // "Hello, Tim!"
```

We added the `sayHi` method *after* the object was declared -
and now it's a full-fledged method of the object.

---

## You can add arrow functions (but be careful)

```javascript
user.sayBye = () => {
  console.log(`Bye, ${user.name}!`);
};

user.sayBye(); // "Bye, Tim!"
```

The difference:

- `function()` has its own `this` (it points to the object itself when called as `user.sayHi()`);
- an arrow function `=>` **has no own** `this`,
so inside it `this` will not refer to the object.

Regular functions, not arrow functions, are usually used for methods.

---

## Example with several methods

```javascript
const calculator = {};

calculator.add = function(a, b) {
  return a + b;
};

calculator.multiply = function(a, b) {
  return a * b;
};

console.log(calculator.add(2, 3));      // 5
console.log(calculator.multiply(2, 3)); // 6
```

Methods can be added any number of times, at any point.

---

## You can add them via `Object.assign()`

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

Object.assign(user, {
  sayHi() {
    console.log(`Hello, ${this.name}`);
  },
  sayBye() {
    console.log(`Bye, ${this.name}`);
  }
});

user.sayHi();  // "Hello, Tim"
user.sayBye(); // "Bye, Tim"
```

Convenient when you need to add several methods at once.

---

## You can add a method to the prototype (it spreads to all objects)

```javascript
function User(name) {
  this.name = name;
}

User.prototype.sayHi = function() {
  console.log(`Hello, ${this.name}`);
};

const tim = new User('Tim');
const oleh = new User('Oleh');

tim.sayHi();  // "Hello, Tim"
oleh.sayHi(); // "Hello, Oleh"
```

This approach adds the method to **all instances** created via `new User()`.

---

## SUMMARY

| Method | When to use | Example |
|---|---|---|
| `obj.method = function() {}` | Add a single method | `user.sayHi = ...` |
| `Object.assign(obj, { ... })` | Add several methods | `Object.assign(user, { sayHi, sayBye })` |
| `prototype.method = ...` | Add a method to all instances | `User.prototype.sayHi = ...` |

---

**In one phrase:**

> Yes, in JavaScript you can add methods to an object at any time -
> just assign it a function, and it becomes a method:
>
> ```javascript
> obj.newMethod = function() { ... };
> ```

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