Interview Questions on API Testing Using Postman

Introduction – Why API Testing Using Postman Is Important in Interviews

In modern applications, APIs connect frontend, backend, mobile apps, and third-party services. Because of this, most QA and automation interviews now include interview questions on API testing using Postman.

Interviewers ask these questions to check whether you:

• Understand API concepts beyond UI testing
  
• Can manually test APIs using Postman
  
• Know how to validate status codes, headers, and response bodies
  
• Can write basic test scripts in Postman
  
• Can think in real-time scenarios, not just theory
  

For freshers, Postman is often the entry point into API testing. For experienced testers, interviewers expect advanced usage like collections, environments, chaining, and automation with Newman.

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What Is API Testing? (Short & Simple)

API testing is the process of validating APIs to ensure they:

• Return correct responses
  
• Handle valid and invalid inputs properly
  
• Are secure, fast, and reliable
  

Simple Example

If you test a login API in Postman:

• Valid credentials → 200 OK + token
  
• Invalid credentials → 401 Unauthorized
  

API testing focuses on data and business logic, not UI.

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REST vs SOAP vs GraphQL (Interview Comparison)

Feature	REST	SOAP	GraphQL
Protocol	HTTP	XML-based	HTTP
Data Format	JSON / XML	XML only	JSON
Performance	Fast	Slower	Optimized
Flexibility	High	Low	Very High
Common Usage	Web & mobile apps	Banking/legacy	Modern APIs

Most Postman testing questions focus on REST APIs.

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Interview Questions on API Testing Using Postman (80+ Q&A)

Section 1: Postman Basics (Q1–Q20)

1. What is Postman?

Postman is a tool used to design, send, and test APIs manually and automatically.

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2. Why is Postman used in API testing?

It simplifies API testing without writing code and supports automation through scripts.

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3. What types of APIs can be tested using Postman?

• REST APIs
  
• SOAP APIs
  
• GraphQL APIs
  

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4. What is a collection in Postman?

A collection is a group of related API requests.

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5. What is an environment in Postman?

An environment stores variables like base URL, tokens, and credentials.

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6. What are Postman variables?

Dynamic values used in requests.

Example:

{{base_url}}

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7. Difference between global and environment variables?

• Global → accessible everywhere
  
• Environment → specific to an environment
  

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8. What is request in Postman?

An API call with method, URL, headers, and body.

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9. What is response in Postman?

Data returned by the API after request execution.

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10. What HTTP methods does Postman support?

GET, POST, PUT, PATCH, DELETE, HEAD, OPTIONS

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11. What is GET request?

Used to retrieve data.

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12. What is POST request?

Used to create data.

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13. What is PUT vs PATCH?

• PUT → full update
  
• PATCH → partial update
  

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14. What is DELETE request?

Used to remove data.

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15. What is request body?

Data sent to server (JSON/XML).

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16. What is response body?

Data returned by server.

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17. What is header in Postman?

Metadata like Content-Type and Authorization.

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18. What is Authorization tab in Postman?

Used to configure authentication (Bearer, Basic, OAuth).

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19. What is Bearer Token?

Token-based authentication mechanism.

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20. Can Postman test SOAP APIs?

Yes, using XML payloads.

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Section 2: Status Codes & Validation (Q21–Q35)

Status Code	Meaning
200	OK
201	Created
204	No Content
400	Bad Request
401	Unauthorized
403	Forbidden
404	Not Found
409	Conflict
500	Server Error

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21. What is 200 status code?

Request successful.

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22. What is 201?

Resource created successfully.

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23. What is 204?

Success without response body.

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24. What is 400?

Invalid client request.

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25. What is 401?

Authentication failed.

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26. What is 403?

Access denied.

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27. What is 404?

Resource not found.

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28. What is 409?

Conflict, usually duplicate data.

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29. What is 500?

Internal server error.

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30. Is status code 200 always correct?

No. Data may still be incorrect.

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31. What validations do you perform in Postman?

• Status code
  
• Response body
  
• Headers
  
• Response time
  

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32. What is response time validation?

Checking API performance.

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33. What is schema validation?

Validating structure and data types.

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34. What is content-type validation?

Ensuring correct response format.

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35. What is header validation?

Checking headers like Authorization and Cache-Control.

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API Validation Example Using Postman

Sample Request

POST /api/login

Content-Type: application/json

{

  “username”: “testuser”,

  “password”: “pass123”

}

Sample Response

{

  “token”: “abc.def.123”,

  “expires_in”: 3600

}

Validations

• Status code = 200
  
• Token is not null
  
• Expiry value > 0
  

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Postman Test Scripts (Interview Favorite)

pm.test(“Status code is 200”, function () {

    pm.response.to.have.status(200);

});

pm.test(“Token exists”, function () {

    var jsonData = pm.response.json();

    pm.expect(jsonData.token).to.not.be.undefined;

});

pm.test(“Response time is less than 2000ms”, function () {

    pm.expect(pm.response.responseTime).to.be.below(2000);

});

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Advanced Postman Interview Questions (Q36–Q60)

36. What is Pre-request Script?

Script executed before request is sent.

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37. Why use Pre-request Script?

To generate tokens, timestamps, or dynamic data.

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38. What is API chaining in Postman?

Using one API’s response in another API.

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39. How do you store token from response?

Using environment variables.

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40. What is Newman?

Command-line tool to run Postman collections.

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41. Can Postman be used for automation?

Yes, with collections + Newman.

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42. What is data-driven testing in Postman?

Running APIs with multiple data sets.

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43. What file formats Postman supports for data?

CSV and JSON.

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44. What is Postman Collection Runner?

Runs collection multiple times.

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45. What is assertion in Postman?

Validation condition.

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46. How do you validate JSON fields?

Using pm.response.json().

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47. How do you validate array size?

Using length assertion.

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48. Can Postman validate XML?

Yes, using xml2Json().

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49. What is environment switching?

Testing APIs in different environments.

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50. What is API mocking in Postman?

Simulating API responses.

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51. What is negative API testing?

Testing invalid inputs.

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52. What is boundary testing?

Testing min and max values.

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53. What is pagination testing?

Validating page-wise data.

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54. What is rate limit testing?

Validating request limits.

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55. What is authentication testing?

Testing valid and invalid credentials.

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56. What is authorization testing?

Testing access control.

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57. What is API regression testing?

Re-testing APIs after changes.

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58. What is API smoke testing?

Basic health check of APIs.

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59. What is logging in Postman?

Capturing request and response.

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60. What are common Postman interview mistakes?

Only checking status code.

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Scenario-Based Interview Questions on API Testing Using Postman (15)

1. API returns 200 but wrong data – what do you validate?
   
2. Login API works, profile API fails – possible reasons?
   
3. Token expired but API still accessible – defect?
   
4. API works in Postman but not in UI – why?
   
5. API fails only in production – what do you check?
   
6. Duplicate records created – what status code expected?
   
7. Unauthorized user accessing secured API – issue?
   
8. API response is slow – how do you test?
   
9. API crashes for special characters – what testing?
   
10. Missing fields in response – what validation?
    
11. API returns null values – how to handle?
    
12. Same request returns different responses – why?
    
13. Payment deducted but order not created – what testing?
    
14. API returns XML instead of JSON – issue?
    
15. API works locally but fails in Newman – reason?
    

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How Interviewers Evaluate Your Answers

Interviewers look for:

• Clear understanding of Postman basics
  
• Ability to validate responses, not just status codes
  
• Real-time scenario thinking
  
• Practical knowledge of scripts and variables
  

Hands-on explanation > memorized answers

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Postman API Testing Interview Cheatsheet

• Always validate response body
  
• Never trust only 200 status
  
• Use environment variables
  
• Write Postman test scripts
  
• Test positive & negative cases
  
• Understand API chaining
  

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FAQs – Interview Questions on API Testing Using Postman

Q1. Is Postman enough for API testing interviews?
Yes, for manual API testing and basics.

Q2. Do interviewers expect automation in Postman?
Basic scripting and Newman knowledge is a plus.

Q3. Should freshers learn Postman first?
Yes, it’s the best starting point.

Q4. REST or SOAP – which is more important?
REST is more common.

Q5. Biggest mistake in Postman interviews?
Not validating response data.