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Implement lodash.set(obj, path, value) — create nested path

Q: Implement lodash.set(obj, path, value) — create nested path

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the real, easy-to-get-wrong detail this question tests — that a NUMERIC path segment should create a genuine ARRAY, not a plain object with a stringified numeric key — was verified directly: `set({}, "a[0].b", "x")` correctly produced a real array at `a` (confirmed via `Array.isArray`), not a plain object shaped like `{"0": {...}}`. ## 1. The problem, restated Write `value` at the end of a deeply nested path, MUTATING the original object in place — automatically creating any MISSING intermediate container along the way (a plain object for a string key segment, but a genuine ARRAY specifically when the NEXT segment looks numeric) — matching lodash's own well-known `_.set` contract, including its real, defining array-vs-object creation heuristic. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Creat

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The problem

Problem, as an interviewer would state it: "Implement set(obj, path, value) — writing a value at a deeply nested path, automatically CREATING any missing intermediate objects (or arrays, for numeric path segments) along the way, matching lodash's own well-known _.set behavior."

Examples:

Code

set({}, "a.b.c", 42); // {a: {b: {c: 42}}}

Clarifying questions expected:

Does this need to CREATE missing intermediate objects, or only update a path that already fully exists?
Should a numeric path segment (like "a[0].b") create a real ARRAY at that level, not a plain object with a "0" key?
Does this mutate the original object in place, or return a genuinely new one?

Code / implementation expected: Yes — real, direct proof of missing-intermediate-object creation, correct updates to an already-existing deep path, and correct array creation for a numeric path segment.

set
lodash-set
path

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Solution

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

How to read this doc: the real, easy-to-get-wrong detail this question tests — that a NUMERIC path segment should create a genuine ARRAY, not a plain object with a stringified numeric key — was verified directly: set({}, "a[0].b", "x") correctly produced a real array at a (confirmed via Array.isArray), not a plain object shaped like {"0": {...}}.

1. The problem, restated

Write value at the end of a deeply nested path, MUTATING the original object in place — automatically creating any MISSING intermediate container along the way (a plain object for a string key segment, but a genuine ARRAY specifically when the NEXT segment looks numeric) — matching lodash's own well-known _.set contract, including its real, defining array-vs-object creation heuristic.

2.

The solution is hidden so you can try first

7 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: set() correctly creates missing intermediate objects, updates existing deep paths, and creates a real array (not a plain object) for a numeric path segment

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