226 lines
6.4 KiB
Markdown
226 lines
6.4 KiB
Markdown
# Object copying, references
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One of the fundamental differences of objects vs primitives is that they are stored and copied "by reference".
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Primitive values: strings, numbers, booleans -- are assigned/copied "as a whole value".
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For instance:
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```js
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let message = "Hello!";
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let phrase = message;
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```
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As a result we have two independent variables, each one is storing the string `"Hello!"`.
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Objects are not like that.
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**A variable stores not the object itself, but its "address in memory", in other words "a reference" to it.**
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Here's the picture for the object:
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```js
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let user = {
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name: "John"
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};
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```
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Here, the object is stored somewhere in memory. And the variable `user` has a "reference" to it.
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**When an object variable is copied -- the reference is copied, the object is not duplicated.**
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For instance:
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```js no-beautify
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let user = { name: "John" };
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let admin = user; // copy the reference
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```
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Now we have two variables, each one with the reference to the same object:
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We can use any variable to access the object and modify its contents:
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```js run
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let user = { name: 'John' };
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let admin = user;
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*!*
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admin.name = 'Pete'; // changed by the "admin" reference
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*/!*
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alert(*!*user.name*/!*); // 'Pete', changes are seen from the "user" reference
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```
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The example above demonstrates that there is only one object. As if we had a cabinet with two keys and used one of them (`admin`) to get into it. Then, if we later use another key (`user`) we can see changes.
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## Comparison by reference
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The equality `==` and strict equality `===` operators for objects work exactly the same.
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**Two objects are equal only if they are the same object.**
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Here two variables reference the same object, thus they are equal:
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```js run
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let a = {};
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let b = a; // copy the reference
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alert( a == b ); // true, both variables reference the same object
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alert( a === b ); // true
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```
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And here two independent objects are not equal, even though both are empty:
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```js run
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let a = {};
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let b = {}; // two independent objects
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alert( a == b ); // false
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```
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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 occur very rarely, usually as a result of a coding mistake.
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## Cloning and merging, Object.assign
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So, copying an object variable creates one more reference to the same object.
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But what if we need to duplicate an object? Create an independent copy, a clone?
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That's also doable, but a little bit more difficult, because there's no built-in method for that in JavaScript. Actually, that's rarely needed. Copying by reference is good most of the time.
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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.
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Like this:
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```js run
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let user = {
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name: "John",
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age: 30
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};
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*!*
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let clone = {}; // the new empty object
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// let's copy all user properties into it
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for (let key in user) {
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clone[key] = user[key];
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}
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*/!*
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// now clone is a fully independent object with the same content
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clone.name = "Pete"; // changed the data in it
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alert( user.name ); // still John in the original object
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```
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Also we can use the method [Object.assign](mdn:js/Object/assign) for that.
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The syntax is:
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```js
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Object.assign(dest, [src1, src2, src3...])
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```
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- The first argument `dest` is a target object.
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- Further arguments `src1, ..., srcN` (can be as many as needed) are source objects.
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- 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.
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- The call returns `dest`.
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For instance, we can use it to merge several objects into one:
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```js
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let user = { name: "John" };
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let permissions1 = { canView: true };
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let permissions2 = { canEdit: true };
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*!*
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// copies all properties from permissions1 and permissions2 into user
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Object.assign(user, permissions1, permissions2);
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*/!*
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// now user = { name: "John", canView: true, canEdit: true }
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```
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If the copied property name already exists, it gets overwritten:
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```js run
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let user = { name: "John" };
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Object.assign(user, { name: "Pete" });
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alert(user.name); // now user = { name: "Pete" }
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```
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We also can use `Object.assign` to replace `for..in` loop for simple cloning:
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```js
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let user = {
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name: "John",
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age: 30
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};
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*!*
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let clone = Object.assign({}, user);
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*/!*
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```
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It copies all properties of `user` into the empty object and returns it.
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## Nested cloning
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Until now we assumed that all properties of `user` are primitive. But properties can be references to other objects. What to do with them?
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Like this:
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```js run
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let user = {
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name: "John",
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sizes: {
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height: 182,
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width: 50
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}
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};
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alert( user.sizes.height ); // 182
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```
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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:
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Like this:
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```js run
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let user = {
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name: "John",
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sizes: {
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height: 182,
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width: 50
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}
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};
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let clone = Object.assign({}, user);
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alert( user.sizes === clone.sizes ); // true, same object
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// user and clone share sizes
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user.sizes.width++; // change a property from one place
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alert(clone.sizes.width); // 51, see the result from the other one
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```
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To fix that, we should use the 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".
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We can use recursion to implement it. Or, not to reinvent the wheel, take an existing implementation, for instance [_.cloneDeep(obj)](https://lodash.com/docs#cloneDeep) from the JavaScript library [lodash](https://lodash.com).
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## Summary
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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.
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All operations via copied references (like adding/removing properties) are performed on the same single object.
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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)](https://lodash.com/docs#cloneDeep).
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