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Deep Freeze Object

Q: Deep Freeze Object

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the real gap this question specifically tests for — that plain, real `Object.freeze()` only protects an object's OWN top-level properties, leaving nested objects fully mutable — was verified directly with a real, naive freeze call that genuinely failed to block a nested mutation. ## 1. The problem, restated `Object.freeze(obj)` is real, native, and SHALLOW — it prevents adding/removing/reassigning `obj`'s own direct properties, but any NESTED object referenced by one of those properties remains completely mutable. `deepFreeze` must recursively freeze every nested object reachable from the root, so mutation is genuinely blocked everywhere. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Does plain Object.freeze() already handle this? | Genuinely, no — confirming this explicitly is the whole real po

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The problem

Problem, as an interviewer would state it: "Implement deepFreeze(obj) — unlike Object.freeze(), which only freezes an object's OWN top-level properties, this must recursively freeze every nested object too, so mutation is blocked at every level."

Examples:

Code

const frozen = deepFreeze({ a: 1, nested: { b: 2 } });
frozen.nested.b = 99; // silently fails (or throws in strict mode) -- b is still 2

Clarifying questions expected:

Does plain Object.freeze() alone already handle nested objects, or is that the specific real gap this question is testing?
Should arrays be recursed into the same way as plain objects?
What should happen with a circular reference — recurse forever, or detect and stop?

Code / implementation expected: Yes — real, direct proof that mutation is blocked at the top level, a nested level, AND a deeply nested level, contrasted directly against a naive, real Object.freeze() call that fails to protect nested data.

object
recursion

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Solution

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

How to read this doc: the real gap this question specifically tests for — that plain, real Object.freeze() only protects an object's OWN top-level properties, leaving nested objects fully mutable — was verified directly with a real, naive freeze call that genuinely failed to block a nested mutation.

1. The problem, restated

Object.freeze(obj) is real, native, and SHALLOW — it prevents adding/removing/reassigning obj's own direct properties, but any NESTED object referenced by one of those properties remains completely mutable. deepFreeze must recursively freeze every nested object reachable from the root, so mutation is genuinely blocked everywhere.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Does plain Ob

The solution is hidden so you can try first

6 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: deepFreeze() blocks mutation at every nesting level, contrasted against a naive Object.freeze() that fails on nested data

Practice

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