diff --git a/1-js/04-object-basics/02-object-copy/article.md b/1-js/04-object-basics/02-object-copy/article.md index 91de5350..68b4dd5d 100644 --- a/1-js/04-object-basics/02-object-copy/article.md +++ b/1-js/04-object-basics/02-object-copy/article.md @@ -39,11 +39,11 @@ The object is stored somewhere in memory (at the right of the picture), while th We may think of an object variable, such as `user`, as like a sheet of paper with the address. -When we perform actions with the object, e.g. take a property `user.name`, JavaScript engine looks at what's at that address and performs the operation on the actual object. +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 is not duplicated.** +**When an object variable is copied, the reference is copied, but the object itself is not duplicated.** For instance: @@ -53,13 +53,13 @@ let user = { name: "John" }; let admin = user; // copy the reference ``` -Now we have two variables, each one with the reference to the same object: +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 any variable to access the object and modify its contents: +We can use either variable to access the object and modify its contents: ```js run let user = { name: 'John' }; @@ -73,7 +73,7 @@ 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 see the changed content. +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 @@ -106,7 +106,7 @@ 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. Actually, that's rarely needed. Copying by reference is good most of the time. +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. @@ -225,7 +225,7 @@ 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 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". +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)](https://lodash.com/docs#cloneDeep) from the JavaScript library [lodash](https://lodash.com). @@ -248,7 +248,7 @@ 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, and that's all. +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 . ````