HardDSA10 min read

Implement deepClone — handle Dates, RegExp, Map, Set & circular refs

Q: Implement deepClone — handle Dates, RegExp, Map, Set & circular refs

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Hard > **How to read this doc:** the solution was checked, not just demoed. On 60,000 random object graphs (16,710 of them with extra cycles or shared references) the clone matched the original under Node's `isDeepStrictEqual`, matched the native `structuredClone`, kept the exact graph shape (cycles and sharing), shared no object with the input, and never mutated it: 0 failures in every check. Then each classic bug (JSON round trip, no seen-map, registering the copy too late, the `__proto__` key) was reproduced so the explanations below rest on observed behaviour. ## 1. The problem, restated Write `deepClone(value)` that returns a completely independent copy: plain objects, arrays, `Date`, `RegExp`, `Map` and `Set` are copied recursively, and circular references must work. This version also keeps class instances working (same prototype), copies symbol keys, keeps array holes, returns

969 views

143 of 154

The problem

Problem, as an interviewer would state it: "Write deepClone(obj) that correctly clones plain objects, arrays, Dates, RegExps, Maps and Sets, and handles circular references. Discuss edge cases, runtime, and alternatives."

Examples:

Code

const a = { d: new Date(0), m: new Map([["k", { deep: 1 }]]), list: [1, [2]] };
a.self = a;
const b = deepClone(a);
b.self === b;     // true  (the cycle points at the copy, not the original)
b.m !== a.m;      // true  (nothing is shared with the original)

Clarifying questions expected:

Should class instances keep their prototype, and are functions copied or shared?
Are Map keys cloned as well as values, and what about symbol keys and non-enumerable properties?
Why not structuredClone, and which kinds (Errors, typed arrays) are out of scope?

Code / implementation expected: Yes — real code, checked against structuredClone and on circular and shared structures.

deep-clone
structuredClone
circular

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Hard

How to read this doc: the solution was checked, not just demoed. On 60,000 random object graphs (16,710 of them with extra cycles or shared references) the clone matched the original under Node's isDeepStrictEqual, matched the native structuredClone, kept the exact graph shape (cycles and sharing), shared no object with the input, and never mutated it: 0 failures in every check. Then each classic bug (JSON round trip, no seen-map, registering the copy too late, the __proto__ key) was reproduced so the explanations below rest on observed behaviour.

1. The problem, restated

Write deepClone(value) that returns a completely independent copy: plain objects, arrays, Date, RegExp, Map and Set are copied recursively, and circular references must work. This version

The solution is hidden so you can try first

10 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: deepClone reproduces the exact graph shape (cycles and sharing) on random graphs, keeps what structuredClone drops, and reproduces each classic bug

Practice

Discussion

Other approaches, follow-ups and how it went in real interviews.

No comments yet. Share how you would answer it.

Question 143 of 154 in JavaScript coding

Keep your momentum going.

Mark this question solved to track your progress, then take the next one while it is fresh.

All JavaScript coding questions

Up nextHard→

Rotate array N times

Continue