What APIs are and why testing them matters in Postman - Build an Automation Script

Verify GET API returns correct user data

Preconditions (2)

Step 1: Send a GET request to https://api.example.com/users/123

Step 2: Check the response status code is 200

Step 3: Verify the response body contains user ID 123

Step 4: Verify the response body contains user name 'John Doe'

Step 5: Verify the response body contains email '[e-mail]'

✅ Expected Result: The API returns status 200 and the correct user data in JSON format

Automation Requirements - Postman

Assertions Needed:

Status code is 200

Response JSON contains correct user ID

Response JSON contains correct user name

Response JSON contains correct email

Best Practices:

Use environment variables for API base URL

Use test scripts in Postman to assert response data

Keep tests independent and repeatable

Validate both status code and response body content

Automated Solution

Postman

pm.test('Status code is 200', function () {
    pm.response.to.have.status(200);
});

const responseJson = pm.response.json();
pm.test('User ID is 123', function () {
    pm.expect(responseJson.id).to.eql(123);
});

pm.test('User name is John Doe', function () {
    pm.expect(responseJson.name).to.eql('John Doe');
});

pm.test('User email is [e-mail]', function () {
    pm.expect(responseJson.email).to.eql('[e-mail]');
});

This Postman test script checks the API response after sending a GET request.

First, it verifies the status code is 200, meaning the request succeeded.

Then, it parses the JSON response and checks that the user ID, name, and email match the expected values.

Using pm.test and pm.expect ensures clear, readable assertions that will show pass or fail in the Postman test results.

This approach helps confirm the API returns correct data and is working as expected.

Common Mistakes - 3 Pitfalls

Not checking the status code before validating response data

Hardcoding API URLs directly in tests

Not parsing JSON response before accessing fields

Bonus Challenge

Now add data-driven testing with 3 different user IDs and verify their data

Show Hint

Practice

1. What is an API in simple terms?

easy

A. A programming language

B. A type of computer virus

C. A way for different software to talk to each other

D. A hardware device

Solution

1. Step 1: Understand the role of APIs

   APIs allow different software programs to communicate and share data.

2. Step 2: Eliminate incorrect options

   Options A, B, and D describe unrelated concepts like programming languages, viruses, or hardware.

3. Final Answer:

   A way for different software to talk to each other -> Option C

4. Quick Check:

   API = software communication [OK]

Hint: APIs connect software systems to share data [OK]

Common Mistakes:

• Confusing API with programming language
• Thinking API is hardware
• Believing API is a virus

2. Which of the following is the correct way to send a GET request in Postman?

easy

A. Write GET in the body and click Send

B. Select GET method, enter URL, click Send

C. Select POST method, enter URL, click Send

D. Enter URL in headers and click Send

Solution

1. Step 1: Identify the correct HTTP method usage

   GET requests are sent by selecting GET method, entering the URL, and clicking Send in Postman.

2. Step 2: Check incorrect options

   Write GET in the body and click Send wrongly puts GET in the body; C uses POST method; D misuses headers for URL.

3. Final Answer:

   Select GET method, enter URL, click Send -> Option B

4. Quick Check:

   GET request = method + URL + Send [OK]

Hint: GET requests need method, URL, then Send [OK]

Common Mistakes:

• Putting method name in body
• Using wrong HTTP method
• Entering URL in headers

3. In Postman, if you send a request and receive status code 404, what does it mean?

medium

A. The requested resource was not found on the server

B. The server is working fine and returned data

C. The request was successful with no content

D. The server encountered an internal error

Solution

1. Step 1: Understand HTTP status codes

   Status code 404 means the server could not find the requested resource (page or data).

2. Step 2: Match code to meaning

   The requested resource was not found on the server correctly describes 404; B describes success (200 OK); C is 204; D is 500 error.

3. Final Answer:

   The requested resource was not found on the server -> Option A

4. Quick Check:

   404 = Not Found [OK]

Hint: 404 means resource missing on server [OK]

Common Mistakes:

• Confusing 404 with success codes
• Thinking 404 means server error
• Mixing 404 with no content status

4. You sent a POST request in Postman but got a 400 Bad Request error. What is the most likely cause?

medium

A. The request body is missing or malformed

B. The server is down

C. The URL is correct but method is GET

D. The response data is too large

Solution

1. Step 1: Understand 400 Bad Request meaning

   400 error means the server could not understand the request due to bad syntax or missing data.

2. Step 2: Identify likely cause in POST request

   POST requests usually require a body; if missing or malformed, server returns 400.

3. Final Answer:

   The request body is missing or malformed -> Option A

4. Quick Check:

   400 = Bad Request due to bad input [OK]

Hint: 400 error means bad or missing request data [OK]

Common Mistakes:

• Assuming server is down for 400 error
• Confusing method mismatch with 400
• Thinking response size causes 400

5. Why is it important to test APIs using tools like Postman before integrating them into an application?

hard

A. To avoid writing any code in the application

B. To make the API run faster on the server

C. Because APIs never change once created

D. To ensure the API responds correctly and handles errors gracefully

Solution

1. Step 1: Understand the purpose of API testing

   Testing APIs ensures they return correct data and handle errors well before using them in apps.

2. Step 2: Evaluate incorrect options

   A is wrong because testing doesn't replace coding; C is false as APIs can change; D is unrelated to testing.

3. Final Answer:

   To ensure the API responds correctly and handles errors gracefully -> Option D

4. Quick Check:

   API testing = verify correctness and error handling [OK]

Hint: Test APIs to check responses and errors before use [OK]

Common Mistakes:

• Thinking testing replaces coding
• Assuming APIs never change
• Believing testing improves server speed