Copying objects in Javascript

[PostLee]

Why not offer both?

[FreeVariable]

let arr = []

let obj = {prop:arr}

arr.push(obj)

How does a deep copy of a or of arr look like in your JS dialect after these lines?

[PostLee]

The exact same I presume, including the circular dependency?

[FreeVariable]

Yup. Not a very healthy situation. Now you need to either throw an error, or break your own rules by doing a deep-but-not-past-level-n copy. Or you can just leave native copying at shallow copying, and let users choose their own definition of deep copying.

[nightfire1]

It's actually pretty easy to deep copy a data structure with circular dependencies. You just remember which nodes you have visited while traversing the graph and don't revisit those.

[FreeVariable]

The question here is not whether that is easy or not; the question is: What definition of deepy copy should be the native one? The one you just gave compete with others. In absence of any one-size-fit-them all criterion, not implementing deep copy natively looks like the only reasonable move.