Object.create don't do shallow copies... Does?

[bsshenrique]

MDN URL

https://developer.mozilla.org/en-US/docs/Glossary/Deep_copy

What specific section or headline is this issue about?

https://github.com/mdn/content/blob/main/files/en-us/glossary/shallow_copy/index.md?plain=1#L27

What information was incorrect, unhelpful, or incomplete?

"In JavaScript, standard built-in object-copy operations (spread syntax, Array.prototype.concat(), Array.prototype.slice(), Array.from(), Object.assign(), and Object.create()) do not create deep copies (instead, they create shallow copies)."

Object.create() is same as obj1 = obj2 and we can verify this writing something like that:

const original = {
  value: 'X',
  object: {
    value: 'Y'
  }
};

// Copy by reference
const copy1 = original;
const copy2 = Object.create(original);
// Shallow copy
const copy3 = Object.assign({}, original);

// A function to just help us
function print(o) {
  return `value: ${o.value} | object.value: ${o.object.value}`
}

original.value = 'A'
original.object.value = 'B'

console.log(
  '\n',
  `Copy 1: ${print(copy1)}\n`,
  `Copy 2: ${print(copy2)}\n`,
  `Copy 3: ${print(copy3)}\n`,
)

We'll see something like:

 Copy 1: value: A | object.value: B
 Copy 2: value: A | object.value: B
 Copy 3: value: X | object.value: B

As we can see, when using Object.create() the top level value changes.

To be considered a Shallow Copy instruction this should not be happen, according to this page:

For shallow copies, only the top-level properties are copied, not the values of nested objects. Therefore:
Re-assigning top-level properties of the copy does not affect the source object.
Re-assigning nested object properties of the copy does affect the source object.

What did you expect to see?

"In JavaScript, standard built-in object-copy operations (spread syntax, Array.prototype.concat(), Array.prototype.slice(), Array.from(), Object.assign()) do not create deep copies (instead, they create shallow copies)."

Do you have any supporting links, references, or citations?

No response

Do you have anything more you want to share?

No response

MDN metadata

Page report details

• Folder: en-us/glossary/deep_copy
• MDN URL: https://developer.mozilla.org/en-US/docs/Glossary/Deep_copy
• GitHub URL: https://github.com/mdn/content/blob/main/files/en-us/glossary/deep_copy/index.md
• Last commit: 1c285ca
• Document last modified: 2023-08-12T02:08:39.000Z

[Josh-Cena]

Object.create does not create a copy. It creates an object with the passed object as its prototype, which breaks our definition of "copy" by the "structurally equivalent prototype chain" criterion.

I don't know why Object.create is mentioned as a copying technique. Welcoming a PR to remove it.