[lambda_garden]

I've never had the need to clone anything in a real JS code-base, personally.

What's the use-case?

[VPenkov]

It is very frequently needed when you're working with a component framework like React or Vue. Typically leaf components shouldn't mutate properties directly but rather emit a changed version of the original data they receive.

But it's not necessarily related to frameworks; if you're working with complex enough data structures and you're following a functional approach, you'll need to do similar things sooner than later.

[lambda_garden]

I use React.

Why does that require deep clones?

I simply do:

    return {
      ...current
      foo: "bar"
    };

[Gare]

When the caller passes you a deep structure and you want to ensure they don't mutate it afterwards. But I agree, it's seldom needed in application code.

[mcv]

I came across a bug recently where a data structure from Redux, which is immutable, was passed to a GraphQL library that would modify the data it was passed. So we had to make a deep clone.

[nec4b]

Every time you get an object from somewhere, which needs to be preserved and modified at the same time.