Question 1

fs.promises vs fs callbacks vs sync APIs — which in servers?

Accepted Answer

Prefer promises/async in request paths; avoid sync fs (*Sync) on the event loop in production servers. Sync APIs block the loop. Callbacks still work but promises compose better. For CLIs/scripts, sync is sometimes acceptable.

Question 2

How do you safely write a file atomically?

Accepted Answer

Write to a temp file in the same directory, then rename (fs.rename) over the target. Rename is atomic on same filesystem. Avoid partial reads of half-written configs. fsync when durability requires it.

Question 3

What path traversal risks exist with user-provided filenames?

Accepted Answer

Users can pass ../ to escape intended directories if you naively join paths. Resolve and ensure the final path stays under an allowlisted root. Prefer generated IDs over raw names. Never trust Content-Disposition filenames blindly.

Question 4

createReadStream vs readFile for large files?

Accepted Answer

Streaming for large/unknown size; readFile loads entire content into memory. HTTP file serving should stream with proper headers. readFile is fine for small configs. Memory limits make this a production decision.

Question 5

How does the libuv thread pool affect heavy fs workloads?

Accepted Answer

Many fs ops share the default pool (UV_THREADPOOL_SIZE, often 4) — saturation increases latency. Tune pool size carefully, or use dedicated workers/queues. Don't assume unlimited parallel disk I/O. Measure under load.

Question 6

Lead: design a multi-tenant file storage layer on local disk + S3.

Accepted Answer

Abstract storage interface, quarantine uploads, virus scan async, signed URLs for S3, path isolation per tenant. Local disk for temp; durable in object storage. Encrypt at rest. Audit access. Never serve files via unconstrained path joins.

File System Node.js Interview Questions

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