How do I clone an array with nested objects without mutating the original?

[Abigail_L]

I’m trying to write a function that returns a new array, but I keep ending up with references to the original data. I started by using the spread operator to copy the array, and at first it looked fine, but as soon as I mutate something inside the result, the original array changes too.

What’s confusing me is that the array items are objects, sometimes with their own nested objects or arrays. I understand that the spread operator only creates a shallow copy, but I’m not fully clear on where the boundary is and how deep the references actually go.

What’s the cleanest way to clone an array with nested objects so that changes to the clone never affect the original? I’m especially interested in approaches that are practical in real code, not just theoretical examples.

[MilaJ]

I’m trying to write a function that should return a new array, but I keep getting a reference to the original instead. I thought I was using the spread operator correctly for a shallow copy, but mutating my result still affects the source. Is there a nuance with nested objects I’m missing about how to properly clone an array?

[Robert_T]

Yep, spread copies the outer array but not the inner objects. If you do const out = [...src]; out[0].foo = 'bar'; src[0].foo also changes. To truly clone you need to clone each item, e.g. const out = src.map(item => item && typeof item === 'object' ? { ...item } : item); If the items themselves have nested objects or arrays, you have to clone those too.

[Luke47]

I ran into this last week. I thought the spread would be enough, but the inner objects stayed the same refs. So I ended up doing a two pass clone and it still felt fragile when there were arrays inside.

[Larry_H]

I tried JSON.parse(JSON.stringify(arr)) for a deep copy, but that wiped methods and turned Dates into strings. StructuredClone is nicer if available, but not everywhere yet.

[Paul86]

Are you sure the problem is the copy and not a mutation somewhere else? Sometimes a function mutates the original after you think you copied it, and you chase a phantom bug.

[Evelyn_T]

I tried a more explicit approach: const res = arr.map(x => Array.isArray(x) ? x.map(y => y) : (x && typeof x === 'object') ? { ...x } : x); it helped in some cases but felt boilerplate.

[Zachary.J]

Drifting a bit here, but I keep coming back to how I construct state objects. If you create new objects up front instead of reusing, the copies feel safer, even if you still have to watch for nested refs.