Node.js &amp; Express

What is a Node.js Stream and why use one

A stream processes data in chunks over time, so memory stays bounded and work starts before all data arrives; ideal for large files and network IO.

advanced2 min read

JWT storage: localStorage versus HttpOnly cookie

LocalStorage is JS-readable so XSS steals the token; HttpOnly cookies resist XSS theft but reintroduce CSRF, mitigated by SameSite plus CSRF tokens.

advanced2 min read

Prototype pollution: how it works and prevention

Attacker writes to Object.prototype via __proto__ keys in merge/parse code, poisoning all objects; prevent by guarding keys, null-prototype objects, Object.freeze, Map, and patched deps.

Deploying a strict CSP for an Express SPA

Define directives, start in Report-Only to gather violations, then enforce; allow inline code via per-request nonces or hashes plus strict-dynamic instead of unsafe-inline.

Auditing and fixing vulnerable npm dependencies

Run npm audit (or yarn audit) to list advisories, npm audit fix to patch within semver, bump majors deliberately, and lock versions; wire audits into CI.

Input validation versus output encoding

Validation checks input fits expected rules on entry; encoding makes data safe for a specific output context on exit. You need both; encoding is the real anti-XSS control.

Preventing SQL injection with parameterized queries

The flaw is SQL injection; prevent it with parameterized queries/prepared statements (pg $1, mysql2 ?), never string concatenation, so input is data not code.

Explaining and preventing CSRF in Express

CSRF abuses a victim's ambient cookies to forge state-changing requests; the server issues an unpredictable token tied to the session, embeds it in forms, and validates it…

Preventing XSS when rendering user content in templates

The risk is XSS; default to escaped interpolation (EJS <%= %>, Pug #{}) so HTML is encoded, and avoid raw output (<%- %>) for untrusted data.

Purpose of Helmet middleware in Express

Helmet sets safe response headers like X-Content-Type-Options, HSTS, and CSP, mitigating MIME-sniffing, clickjacking, and protocol downgrade.

Testing an async workflow that spans DB and message queue

Assert the DB row, then verify the queue message via a test consumer or spy, polling with a timeout rather than fixed sleeps.

Testing code that calls a third-party API

Intercept at the HTTP boundary (nock) or run a local mock server; cover success, errors, timeouts, and assert request shape.

Managing clean test state across API integration tests

Compare seed-and-truncate, per-test transaction rollback, and in-memory or containerized databases, weighing fidelity, speed, and isolation.

Testing async Promise-returning code in Jest

Return or await the promise; use await expect(...).resolves/rejects, or await the value directly.

Mocking the database layer in Jest unit tests

A live DB makes tests slow, flaky, and order-dependent; use jest.mock on the model so methods return controlled fakes.

Integration testing a POST endpoint with Supertest

Pass the Express app to supertest, send a POST with a body, then assert status 201, the response shape, and the persisted side effect; also test validation failures.

Writing a basic Jest unit test

Import the function, group cases with describe, define each case with it or test, assert with expect and a matcher like toBe, covering normal and edge inputs.

Unit, integration, and E2E tests explained

Unit tests isolate one function with dependencies mocked, integration tests exercise several units together (route plus DB), E2E tests drive the whole running system.

Handling uncaughtException and unhandledRejection

Listen on process for uncaughtException and unhandledRejection, log the error, stop accepting new work, drain in-flight requests, then exit non-zero for a supervisor to restart.