Instacart·Backend Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Instacart backend engineer interview, got a debugging/implementation problem centered on a search filter that was returning unfiltered results. Pretty reasonable problem once you slow down and read all the edge cases carefully.

Questions Asked (1)

You're given a broken book catalog search endpoint that returns the entire catalog regardless of what the user types. Implement the backend filtering function so it correctly returns only books whose title, author, or tags contain the query string (case-insensitive, trimmed), with an optional availability filter.

API & IntegrationsRoot Cause AnalysisAlgorithms & Data Structures

Author's notes

The core logic isn't hard but I almost tripped on the empty query case.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and edge cases (e.g., empty query, null tags, availability filter). Then outline a clean, testable function that normalizes the query and filters the catalog using case-insensitive matching on title, author, and tags, with an optional availability check. Finally, discuss performance considerations and potential optimizations for large catalogs.

Pro tip: Mention that you would write unit tests covering edge cases like empty query, whitespace-only query, and missing fields, and that you'd consider indexing or pre-processing for scalability if the catalog is large.

1. Clarify requirements and edge cases

Ask about expected behavior for empty query, null/undefined fields, and how availability filter should interact with search. Confirm that matching is case-insensitive and trimmed.

2. Design the function signature and data flow

Define the function parameters (catalog array, query string, optional availability boolean) and outline the filtering logic. Consider whether to normalize the query once and reuse it.

3. Implement filtering logic

Write code that trims and lowercases the query, then filters books where title, author, or any tag contains the query (case-insensitive). Apply availability filter if provided.

4. Handle edge cases and validation

Ensure empty query returns all books (or as specified), handle missing fields gracefully, and validate input types. Consider trimming book fields as well.

5. Discuss performance and testing

Mention time complexity (O(n*m) for n books and m fields) and potential optimizations like indexing or using a search library. Outline unit tests for correctness.

Key Points to Mention

Case-insensitive matching using toLowerCase() or localeCompare with sensitivity option
Trimming the query and possibly book fields to avoid whitespace mismatches
Handling null/undefined tags and other fields safely
Optional availability filter as a separate condition
Edge cases: empty query, whitespace-only query, no matches
Performance considerations for large catalogs (e.g., indexing, early termination)

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