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## Short answer

Copying with `Object.assign()`
creates a **shallow copy**.
This means **nested objects and arrays are copied by reference**, not by value.

---

## Example

```javascript
const user = {
  name: 'Bohdan',
  address: {
    city: 'Kyiv',
    zip: 12345
  }
};

const clone = Object.assign({}, user);
clone.address.city = 'Lviv';

console.log(user.address.city); // 'Lviv'
```

Why:

- `Object.assign()` copies only the **first level of properties**;
- the `address` property is an **object**, so a **reference** is copied;
- changing the nested object (`city`) is reflected in the original.

---

## How this looks in memory

```javascript
user.address ─────┐
                   │  (the same object)
clone.address ────┘
```

Both objects (`user` and `clone`) **reference the same** `address`.
So a change inside `address` affects both.

---

## But! Primitive properties are copied normally

```javascript
const user = { name: 'Bohdan', age: 25 };
const clone = Object.assign({}, user);

clone.name = 'Oleh';
console.log(user.name); // "Bohdan" (independent)
```

Only **nested objects or arrays** stay shared.

---

## How to avoid this problem

If you want to make a **deep copy**, use:

### `structuredClone()` (a built-in modern way)

```javascript
const clone = structuredClone(user);
clone.address.city = 'Lviv';
console.log(user.address.city); // "Kyiv"
```

### `JSON.parse(JSON.stringify())` (universal, but with limitations)

```javascript
const clone = JSON.parse(JSON.stringify(user));
```

### `_.cloneDeep()` from Lodash (reliable and compatible with all types)

```javascript
import _ from 'lodash';
const clone = _.cloneDeep(user);
```

---

## SUMMARY

| What `Object.assign()` copies | Behavior |
|---|---|
| Primitive values (`string`, `number`, `boolean`) | copied by value |
| Nested objects and arrays | copied **by reference** |
| Functions | copied by reference |
| Deep structures | not copied independently |

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