# Object references and copying

One of the fundamental differences of objects versus primitives is that objects are stored and copied "by reference", whereas primitive values: strings, numbers, booleans, etc -- are always copied "as a whole value".

That's easy to understand if we look a bit under the hood of what happens when we copy a value.

Let's start with a primitive, such as a string.

Here we put a copy of `message` into `phrase`:

```js
let message = "Hello!";
let phrase = message;
```

As a result we have two independent variables, each one storing the string `"Hello!"`.

![](variable-copy-value.svg)

Quite an obvious result, right?

Objects are not like that.

**A variable assigned to an object stores not the object itself, but its "address in memory" -- in other words "a reference" to it.**

Let's look at an example of such a variable:

```js
let user = {
  name: "John"
};
```

And here's how it's actually stored in memory:

![](variable-contains-reference.svg)

The object is stored somewhere in memory (at the right of the picture), while the `user` variable (at the left) has a "reference" to it.

We may think of an object variable, such as `user`, like a sheet of paper with the address of the object on it.

When we perform actions with the object, e.g. take a property `user.name`, the JavaScript engine looks at what's at that address and performs the operation on the actual object.

Now here's why it's important.

**When an object variable is copied, the reference is copied, but the object itself is not duplicated.**

For instance:

```js no-beautify
let user = { name: "John" };

let admin = user; // copy the reference
```

Now we have two variables, each storing a reference to the same object:

![](variable-copy-reference.svg)

As you can see, there's still one object, but now with two variables that reference it.

We can use either variable to access the object and modify its contents:

```js run
let user = { name: 'John' };

let admin = user;

*!*
admin.name = 'Pete'; // changed by the "admin" reference
*/!*

alert(*!*user.name*/!*); // 'Pete', changes are seen from the "user" reference
```

It's as if we had a cabinet with two keys and used one of them (`admin`) to get into it and make changes. Then, if we later use another key (`user`), we are still opening the same cabinet and can access the changed contents.

## Comparison by reference

Two objects are equal only if they are the same object.

For instance, here `a` and `b` reference the same object, thus they are equal:

```js run
let a = {};
let b = a; // copy the reference

alert( a == b ); // true, both variables reference the same object
alert( a === b ); // true
```

And here two independent objects are not equal, even though they look alike (both are empty):

```js run
let a = {};
let b = {}; // two independent objects

alert( a == b ); // false
```

For comparisons like `obj1 > obj2` or for a comparison against a primitive `obj == 5`, objects are converted to primitives. We'll study how object conversions work very soon, but to tell the truth, such comparisons are needed very rarely -- usually they appear as a result of a programming mistake.

````smart header="Const objects can be modified"
An important side effect of storing objects as references is that an object declared as `const` *can* be modified.

For instance:

```js run
const user = {
  name: "John"
};

*!*
user.name = "Pete"; // (*)
*/!*

alert(user.name); // Pete
```

It might seem that the line `(*)` would cause an error, but it does not. The value of `user` is constant, it must always reference the same object, but properties of that object are free to change.

In other words, the `const user` gives an error only if we try to set `user=...` as a whole.

That said, if we really need to make constant object properties, it's also possible, but using totally different methods. We'll mention that in the chapter <info:property-descriptors>.
````

## Cloning and merging, Object.assign [#cloning-and-merging-object-assign]

So, copying an object variable creates one more reference to the same object.

But what if we need to duplicate an object?

We can create a new object and replicate the structure of the existing one, by iterating over its properties and copying them on the primitive level.

Like this:

```js run
let user = {
  name: "John",
  age: 30
};

*!*
let clone = {}; // the new empty object

// let's copy all user properties into it
for (let key in user) {
  clone[key] = user[key];
}
*/!*

// now clone is a fully independent object with the same content
clone.name = "Pete"; // changed the data in it

alert( user.name ); // still John in the original object
```

We can also use the method [Object.assign](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign).

The syntax is:

```js
Object.assign(dest, src1[, src2, src3...])
```

- The first argument `dest` is a target object.
- Further arguments `src1, ..., srcN` (can be as many as needed) are source objects.
- It copies the properties of all source objects `src1, ..., srcN` into the target `dest`. In other words, properties of all arguments starting from the second are copied into the first object.
- The call returns `dest`.

For instance, we can use it to merge several objects into one:
```js
let user = { name: "John" };

let permissions1 = { canView: true };
let permissions2 = { canEdit: true };

*!*
// copies all properties from permissions1 and permissions2 into user
Object.assign(user, permissions1, permissions2);
*/!*

// now user = { name: "John", canView: true, canEdit: true }
```

If the copied property name already exists, it gets overwritten:

```js run
let user = { name: "John" };

Object.assign(user, { name: "Pete" });

alert(user.name); // now user = { name: "Pete" }
```

We also can use `Object.assign` to replace `for..in` loop for simple cloning:

```js
let user = {
  name: "John",
  age: 30
};

*!*
let clone = Object.assign({}, user);
*/!*
```

It copies all properties of `user` into the empty object and returns it.

There are also other methods of cloning an object, e.g. using the [spread syntax](info:rest-parameters-spread) `clone = {...user}`, covered later in the tutorial.

## Nested cloning

Until now we assumed that all properties of `user` are primitive. But properties can be references to other objects.

Like this:
```js run
let user = {
  name: "John",
  sizes: {
    height: 182,
    width: 50
  }
};

alert( user.sizes.height ); // 182
```

Now it's not enough to copy `clone.sizes = user.sizes`, because `user.sizes` is an object, and will be copied by reference, so `clone` and `user` will share the same sizes:

```js run
let user = {
  name: "John",
  sizes: {
    height: 182,
    width: 50
  }
};

let clone = Object.assign({}, user);

alert( user.sizes === clone.sizes ); // true, same object

// user and clone share sizes
user.sizes.width = 60;    // change a property from one place
alert(clone.sizes.width); // 60, get the result from the other one
```

To fix that and make `user` and `clone` truly separate objects, we should use a cloning loop that examines each value of `user[key]` and, if it's an object, then replicate its structure as well. That is called a "deep cloning" or "structured cloning". There's [structuredClone](https://developer.mozilla.org/en-US/docs/Web/API/structuredClone) method that implements deep cloning.


### structuredClone

The call `structuredClone(object)` clones the `object` with all nested properties.

Here's how we can use it in our example:

```js run
let user = {
  name: "John",
  sizes: {
    height: 182,
    width: 50
  }
};

*!*
let clone = structuredClone(user);
*/!*

alert( user.sizes === clone.sizes ); // false, different objects

// user and clone are totally unrelated now
user.sizes.width = 60;    // change a property from one place
alert(clone.sizes.width); // 50, not related
```

The `structuredClone` method can clone most data types, such as objects, arrays, primitive values.

It also supports circular references, when an object property references the object itself (directly or via a chain or references).

For instance:

```js run
let user = {};
// let's create a circular reference:
// user.me references the user itself
user.me = user;

let clone = structuredClone(user);
alert(clone.me === clone); // true
```

As you can see, `clone.me` references the `clone`, not the `user`! So the circular reference was cloned correctly as well.

Although, there are cases when `structuredClone` fails.

For instance, when an object has a function property:

```js run
// error
structuredClone({
  f: function() {}
});
```

Function properties aren't supported.

To handle such complex cases we may need to use a combination of cloning methods, write custom code or, to not reinvent the wheel, take an existing implementation, for instance [_.cloneDeep(obj)](https://lodash.com/docs#cloneDeep) from the JavaScript library [lodash](https://lodash.com).

## Summary

Objects are assigned and copied by reference. In other words, a variable stores not the "object value", but a "reference" (address in memory) for the value. So copying such a variable or passing it as a function argument copies that reference, not the object itself.

All operations via copied references (like adding/removing properties) are performed on the same single object.

To make a "real copy" (a clone) we can use `Object.assign` for the so-called "shallow copy" (nested objects are copied by reference) or a "deep cloning" function `structuredClone` or use a custom cloning implementation, such as [_.cloneDeep(obj)](https://lodash.com/docs#cloneDeep).
