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MediumDSA6 min read

Flatten and Unflatten a Nested Object (dot notation & arrays)

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The problem

Problem, as an interviewer would state it: "Create flattenObject(obj) producing { "a.b.0.c": value } from a nested object, and unflattenObject to reverse it. Support arrays and escaping."

Examples:

Code
flattenObject({ a: { b: [{ c: 1 }] } }); // { "a.b.0.c": 1 }
unflattenObject({ "a.b.0.c": 1 });       // { a: { b: [{ c: 1 }] } }

Clarifying questions expected:

  • How should a key that itself contains a dot be represented so it survives the round trip?
  • Should numeric path segments rebuild real arrays on the way back?
  • What about empty objects and arrays, null leaves, and untrusted input paths like __proto__.x?

Code / implementation expected: Yes — real, direct proof of a full round trip, of dotted keys, and of the safety guard.

  • object
  • flatten
  • unflatten

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Solution

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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: flatten then unflatten round-trips arrays, nulls, empty containers and a dotted key via escaping, rejects a __proto__ path, and shows the numeric-keys limit
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