Object Copy

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:

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

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

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:

let user = {
  name: "John"
};

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

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, as 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:

let user = { name: "John" };

let admin = user; // copy the reference

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

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:

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:

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):

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.

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? Create an independent copy, a clone?

That's also doable, but a little bit more difficult, because there's no built-in method for that in JavaScript. But there is rarely a need -- copying by reference is good most of the time.

But if we really want that, then we need to 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:

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

Also we can use the method Object.assign for that.

The syntax is:

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:

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:

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:

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 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. What to do with them?

Like this:

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 the user.sizes is an object, it will be copied by reference. So clone and user will share the same sizes:

Like this:

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++;       // change a property from one place
alert(clone.sizes.width); // 51, see the result from the other one

To fix that, 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".

We can use recursion to implement it. Or, to not reinvent the wheel, take an existing implementation, for instance _.cloneDeep(obj) from the JavaScript library lodash.

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>.

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, such as _.cloneDeep(obj).