400 Rest API Interview Questions with Answers 2026
Course Overview
What You'll Learn
- REST Fundamentals & API Design: Constraints, URI structure, and Idempotency.
- Data Handling: Content negotiation, JSON standards, and Serialization.
- API Security: JWT, OAuth 2.0, Rate Limiting, and CORS.
- Optimization: Caching strategies, Pagination, and API Gateways.
- DevOps & Testing: OpenAPI/Swagger, Postman, and Contract Testing.
About This Free Course
REST API javascript interview practice tests Questions and Answers is my comprehensive toolkit designed to bridge the gap between basic theory and the high-level architectural knowledge required by top-tier tech companies. I’ve built this course to help you navigate the nuances of resource modeling, security protocols like OAuth 2.0, and performance optimization without the fluff. Whether you are a developer preparing for a backend role or an architect refining your design skills, I provide deep-dive explanations for every single option to ensure you understand not just the "what," but the "why" behind scalable API development. I focus heavily on real-world scenarios, covering everything from idempotency and versioning to the OWASP API Security Top 10, so you can walk into your interview or exam with the confidence of a seasoned professional.
Exam Domains & Sample Topics
REST Fundamentals & API Design: Constraints, URI structure, and Idempotency.
Data Handling: Content negotiation, JSON standards, and Serialization.
API Security: JWT, OAuth 2.0, Rate Limiting, and CORS.
Optimization: Caching strategies, Pagination, and API Gateways.
DevOps & Testing: OpenAPI/Swagger, Postman, and Contract Testing.
Which of the following HTTP methods is considered both idempotent and safe according to RFC 9110 standards?
A) POST
B) PATCH
C) DELETE
D) GET
E) CONNECT
F) TRACE
Correct Answer: D & F (Note: In standard MCQ, choose D as the primary answer).
Overall Explanation: Safety refers to methods that do not modify the resource state, while idempotency means multiple identical requests have the same effect as a single request.
Option Explanations:
A) Incorrect: POST is neither safe nor idempotent (it creates resources).
B) Incorrect: PATCH is not idempotent; repeated applications can change state differently.
C) Incorrect: DELETE is idempotent but not safe (it modifies state by removing it).
D) Correct: GET is safe (read-only) and idempotent.
E) Incorrect: CONNECT is used for tunneling and is not safe.
F) Correct: TRACE is safe and idempotent as it merely echoes the received request.
When implementing an OAuth 2.0 flow for a Single Page Application (SPA) with no backend, which grant type is currently recommended by best security practices?
A) Implicit Grant
B) Resource Owner Password Credentials
C) Authorization Code Flow with PKCE
D) Client Credentials Flow
E) Refresh Token Flow
F) Device Code Flow
Correct Answer: C
Overall Explanation: Due to security vulnerabilities in the Implicit Flow, the Authorization Code Flow with Proof Key for Code Exchange (PKCE) is now the industry standard for public clients.
Option Explanations:
A) Incorrect: Implicit Grant is deprecated due to token leakage risks in the URL.
B) Incorrect: This requires the user to share their password directly with the app, which is insecure.
C) Correct: PKCE provides a cryptographically strong mechanism to prevent authorization code interception.
D) Incorrect: This is for machine-to-machine communication, not user-facing SPAs.
E) Incorrect: This is used to obtain new access tokens, not for initial authentication.
F) Incorrect: This is designed for input-constrained devices like Smart TVs.
If a client requests a resource representation format that the server does not support (e.g., requesting 'application/xml' when only 'application/json' is available), which HTTP status code should I return?
A) 400 Bad Request
B) 403 Forbidden
C) 404 Not Found
D) 405 Method Not Allowed
E) 406 Not Acceptable
F) 415 Unsupported Media Type
Correct Answer: E
Overall Explanation: Content negotiation is handled via the 'Accept' header; when the server cannot fulfill this, it triggers a 406 error.
Option Explanations:
A) Incorrect: 400 is for generic client-side syntax errors.
B) Incorrect: 403 is for permission issues.
C) Incorrect: 404 means the URI itself does not exist.
D) Incorrect: 405 means the HTTP Verb (like PUT) isn't allowed on that URI.
E) Correct: 406 specifically indicates the server cannot produce a response matching the 'Accept' headers.
F) Incorrect: 415 is used when the client sends a payload format (Content-Type) that the server cannot process.
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