Extracting data from responses in Postman - Build an Automation Script

Extract user ID from API response and verify it

Preconditions (2)

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

Step 2: Wait for the response

Step 3: Extract the 'id' field from the JSON response body

Step 4: Verify that the extracted 'id' equals 123

✅ Expected Result: The 'id' field is extracted successfully and equals 123

Automation Requirements - Postman Tests (JavaScript)

Assertions Needed:

Verify response status code is 200

Verify extracted 'id' field equals 123

Best Practices:

Use pm.response.json() to parse JSON response

Use pm.test() for assertions

Use descriptive test names

Avoid hardcoding values except for verification

Automated Solution

Postman

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

const responseJson = pm.response.json();
const userId = responseJson.id;

pm.test('User ID is 123', () => {
    pm.expect(userId).to.eql(123);
});

The first test checks that the response status code is 200, which means the request was successful.

Then, we parse the JSON response using pm.response.json() to get a JavaScript object.

We extract the id field from the response object and store it in userId.

Finally, we assert that userId equals 123 using pm.expect().to.eql(). This confirms the correct data was returned.

Common Mistakes - 3 Pitfalls

Not parsing the response JSON before accessing fields

Using incorrect assertion syntax like pm.expect(userId == 123)

Not checking response status before extracting data

Bonus Challenge

Now add tests to extract and verify 'username' and 'email' fields from the response

Show Hint

Practice

1. What is the primary purpose of extracting data from API responses in Postman?

easy

A. To reuse data in subsequent API requests

B. To change the API endpoint URL

C. To modify the request headers

D. To delete the response data

Solution

1. Step 1: Understand the role of data extraction

   Extracting data allows you to capture values from one response to use later.

2. Step 2: Connect API requests using extracted data

   This helps chain requests by passing data like tokens or IDs forward.

3. Final Answer:

   To reuse data in subsequent API requests -> Option A

4. Quick Check:

   Extract data = reuse in next requests [OK]

Hint: Extract data to pass info between requests [OK]

Common Mistakes:

• Thinking extraction changes the URL
• Confusing extraction with header modification
• Believing extraction deletes data

2. Which Postman script correctly extracts the value of userId from a JSON response and saves it as an environment variable?

easy

A. let data = pm.response.json(); pm.environment.set('userId', data.userId);

B. pm.response.set('userId', pm.response.json().userId);

C. pm.environment.get('userId', pm.response.json().userId);

D. let userId = pm.response.set('userId');

Solution

1. Step 1: Use pm.response.json() to parse JSON

   This method converts the response body into a JavaScript object.

2. Step 2: Use pm.environment.set() to save variable

   Set the environment variable 'userId' with the extracted value.

3. Final Answer:

   let data = pm.response.json(); pm.environment.set('userId', data.userId); -> Option A

4. Quick Check:

   Parse JSON + set env variable = let data = pm.response.json(); pm.environment.set('userId', data.userId); [OK]

Hint: Use pm.response.json() then pm.environment.set() [OK]

Common Mistakes:

• Using pm.response.set() which doesn't exist
• Using pm.environment.get() to set variables
• Not parsing JSON before accessing properties

3. Given the response body:

{"token": "abc123", "user": {"id": 42}}

What will this Postman script save in the environment variable authToken?

let jsonData = pm.response.json();
pm.environment.set('authToken', jsonData.token);

medium

A. null

B. 42

C. undefined

D. "abc123"

Solution

1. Step 1: Parse the JSON response

   jsonData.token accesses the 'token' key which has value "abc123".

2. Step 2: Set environment variable with token value

   pm.environment.set saves "abc123" as 'authToken'.

3. Final Answer:

   "abc123" -> Option D

4. Quick Check:

   jsonData.token = "abc123" [OK]

Hint: Access exact key from parsed JSON to get value [OK]

Common Mistakes:

• Using user.id instead of token
• Expecting number 42 instead of string token
• Not parsing JSON before accessing token

4. You wrote this Postman test script to extract sessionId from the response:

let data = pm.response.json();
pm.environment.set('sessionId', data.session_id);

But the environment variable sessionId is always empty. What is the likely problem?

medium

A. pm.response.json() does not parse JSON

B. pm.environment.set() cannot save variables

C. The response JSON uses sessionId not session_id

D. You must use pm.collectionVariables.set() instead

Solution

1. Step 1: Check JSON key names carefully

   The script uses 'session_id' but the response likely has 'sessionId' (camelCase).

2. Step 2: Correct key name to match response

   Use data.sessionId to correctly extract the value.

3. Final Answer:

   The response JSON uses sessionId not session_id -> Option C

4. Quick Check:

   Key name mismatch causes empty variable [OK]

Hint: Match JSON keys exactly, including case [OK]

Common Mistakes:

• Assuming pm.environment.set() doesn't work
• Not parsing JSON before accessing keys
• Confusing environment and collection variables

5. You receive this nested JSON response:

{"data": {"users": [{"id": 1, "name": "Alice"}, {"id": 2, "name": "Bob"}]}}

How do you extract and save the name of the second user as a collection variable in Postman?

hard

A. pm.environment.set('secondUserName', pm.response.json().data.users[1].name);

B. let json = pm.response.json(); pm.collectionVariables.set('secondUserName', json.data.users[1].name);

C. let json = pm.response.json(); pm.environment.set('secondUserName', json.data.users[2].name);

D. pm.collectionVariables.set('secondUserName', pm.response.json().users[2].name);

Solution

1. Step 1: Parse the nested JSON response

   Access the array at json.data.users and select index 1 for the second user.

2. Step 2: Save the second user's name as a collection variable

   Use pm.collectionVariables.set with key 'secondUserName' and value json.data.users[1].name.

3. Final Answer:

   let json = pm.response.json(); pm.collectionVariables.set('secondUserName', json.data.users[1].name); -> Option B

4. Quick Check:

   Index 1 in users array = second user name [OK]

Hint: Use zero-based index and correct variable scope [OK]

Common Mistakes:

• Using index 2 instead of 1 for second user
• Mixing environment and collection variables
• Not parsing JSON before accessing nested data