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Elastic N.V.·Software Engineer·Technical Phone Screen·Senior

Senior
Jun 2026

Summary

Elastic N.V. full-stack round focused on a buggy JavaScript flatten implementation. You had to read broken code, find the issues, and fix them live while talking through edge cases. Pretty dense for a single session.

Questions Asked (2)

Q1

You're given a JavaScript flatten(arr) function that recursively flattens nested arrays. Find and fix all the bugs in it.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

The bugs they were fishing for: not recursing into non-array elements, misusing concat so it mutates or doesn't spread correctly, and circular reference handling causing infinite recursion.

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AI HintsAI Generated

Suggested Approach

First, read the code carefully and trace through a few examples to identify logical errors, edge cases, and potential infinite loops. Then, explain each bug and propose a corrected version, discussing trade-offs between recursion and iteration, and how to handle non-array elements and nested arrays properly.

Pro tip: Always test with edge cases like empty arrays, deeply nested arrays, and arrays containing non-array elements to catch subtle bugs. Mention that you'd consider using Array.isArray() for type checking and a stack-based approach to avoid stack overflow for very deep nesting.

1. Understand the intended behavior

Clarify that flatten should recursively flatten nested arrays into a single-level array, preserving order and handling any depth.

2. Trace through the code with examples

Walk through the given code with simple inputs like [1, [2, [3]]] to spot logical errors, such as incorrect recursion or missing base cases.

3. Identify all bugs

List each bug: e.g., using concat incorrectly, not checking if an element is an array, mutating the input array, or causing infinite recursion.

4. Propose fixes and discuss trade-offs

Provide a corrected implementation, explaining choices like using Array.isArray(), recursion vs iteration, and handling edge cases.

5. Test and verify

Mention testing with various inputs including empty arrays, deeply nested arrays, and arrays with non-array elements to ensure correctness.

Key Points to Mention

  • Use Array.isArray() to reliably check if an element is an array.
  • Avoid mutating the input array; create a new array for the result.
  • Consider recursion depth limits and potential stack overflow; an iterative approach with a stack can be safer.
  • Handle edge cases: empty arrays, non-array elements, and deeply nested arrays.
  • Ensure the function works for any level of nesting, not just one level.
  • Discuss time and space complexity: O(n) time where n is total elements, O(d) space for recursion depth d.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Q2

Walk through test cases for the fixed flatten function: empty arrays, deeply nested input, and mixed types.

Algorithms & Data StructuresRoot Cause Analysis
Author's notes

This part felt more like a sanity check than a real question.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the function's contract and expected behavior for edge cases, then systematically walk through each category of test cases (empty, deeply nested, mixed types) with concrete examples. For each case, state the input, expected output, and why it matters, and mention any assumptions about the flattening logic.

Pro tip: Demonstrate awareness of production concerns by discussing how you'd handle circular references and performance for very deep nesting, and suggest property-based testing to catch unexpected edge cases.

1. Clarify the function contract

Ask or state the expected behavior: does flatten remove empty arrays? How are non-array iterables handled? What is the depth parameter's default? This sets the context for all test cases.

2. Test empty and trivial inputs

Cover empty array, array with no nesting, and null/undefined inputs. Verify the function returns an empty array or the original array as appropriate, and handles invalid inputs gracefully.

3. Test deeply nested structures

Use arrays nested multiple levels deep, including empty arrays at various depths, to ensure recursion or iteration correctly flattens all levels. Also test a single deeply nested element.

4. Test mixed types and special values

Include numbers, strings, booleans, objects, null, undefined, and functions within arrays. Verify that non-array values are preserved and not accidentally flattened or coerced.

5. Discuss edge cases and performance

Mention circular references, very large arrays, and deep recursion limits. Suggest how to test these (e.g., using a Set to detect cycles, or iterative approach) and the expected behavior.

Key Points to Mention

  • Empty array input should return an empty array; nested empty arrays should be removed.
  • Deeply nested arrays require recursion or an explicit stack; test with at least 3-4 levels and consider stack overflow for extreme depth.
  • Mixed types: ensure non-array elements (including objects, null, undefined) are kept as-is and not flattened.
  • Sparse arrays: decide whether holes are preserved or removed, and test accordingly.
  • Circular references: if the function doesn't handle them, it may infinite loop; test with a self-referencing array.
  • Performance: for large inputs, consider time/space complexity and test with large arrays to ensure no quadratic behavior.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.