Shallow vs deep copy

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Mutability

Copying a container that holds other mutable objects raises a question: copy just the outer box, or everything inside too?

• Shallow copy: a new outer container, but the items inside are the same objects as before.
• Deep copy: new copies of everything, recursively.

How to make each

	Shallow	Deep
Python	lst.copy(), list(lst), lst[:], dict.copy(), copy.copy(x)	copy.deepcopy(x)
JavaScript	[...arr], {...obj}, Object.assign({}, obj)	structuredClone(obj)

Example

import copy
config = {"retries": 3, "tags": ["a"]}
s = config.copy()
d = copy.deepcopy(config)
s["tags"].append("b")
print(config["tags"])   # ['a', 'b']  the shallow copy shares the inner list
print(d["tags"])        # ['a']

const config = { retries: 3, tags: ['a'] };
const s = { ...config };
const d = structuredClone(config);
s.tags.push('b');
console.log(config.tags, d.tags);   // [ 'a', 'b' ] [ 'a' ]

Traps

• A shallow copy of a flat list of numbers or strings is enough: those are immutable.
• JSON.parse(JSON.stringify(x)) as a deep copy loses dates, undefined, Map, Set and functions. Prefer structuredClone.
• Deep copies cost time and memory. Copy only what you will change.

Check yourself

You shallow-copy a list of lists and change copy[0][0]. What happens to the original?

It changes too: both outer lists hold the same inner lists.

What is the modern built-in way to deep-copy in JavaScript?

structuredClone(value).