Setting variables from response in Postman - Build an Automation Script

Set environment variable from API response

Preconditions (3)

Step 1: Send a GET request to the API endpoint https://api.example.com/user/123

Step 2: Wait for the response

Step 3: In the Tests tab, write a script to extract the 'userId' field from the JSON response

Step 4: Set an environment variable named 'userId' with the extracted value

Step 5: Verify that the environment variable 'userId' is set correctly

✅ Expected Result: The environment variable 'userId' is set with the value from the response JSON's 'userId' field

Automation Requirements - Postman Tests (JavaScript)

Assertions Needed:

Assert response status code is 200

Assert response body contains 'userId'

Assert environment variable 'userId' is set and equals the response value

Best Practices:

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

Use pm.environment.set() to set environment variables

Use pm.test() for assertions

Check response status before extracting data

Automated Solution

Postman

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

const responseJson = pm.response.json();

pm.test('Response has userId field', function () {
    pm.expect(responseJson).to.have.property('userId');
});

const userId = responseJson.userId;
pm.environment.set('userId', userId);

pm.test('Environment variable userId is set correctly', function () {
    const envUserId = pm.environment.get('userId');
    pm.expect(envUserId).to.eql(userId);
});

This script first checks that the response status code is 200 to ensure the request succeeded.

Then it parses the JSON response using pm.response.json().

It asserts that the response contains the userId property.

Next, it extracts the userId value and sets it as an environment variable using pm.environment.set().

Finally, it verifies that the environment variable was set correctly by comparing it to the extracted value.

This approach ensures the variable is only set if the response is valid and contains the expected data.

Common Mistakes - 3 Pitfalls

Not checking response status before extracting data

{'mistake': 'Using pm.variables.set() instead of pm.environment.set() for environment variables', 'why_bad': "pm.variables.set() sets variables only for the current request scope, not environment scope, so the variable won't persist.", 'correct_approach': 'Use pm.environment.set() to set environment variables that persist across requests.'}

Not verifying that the response contains the expected property before setting variable

Bonus Challenge

Now add data-driven testing by setting variables from three different API endpoints with different user IDs

Show Hint

Practice

1. What is the main purpose of setting variables from a response in Postman?

easy

A. To encrypt the response data for security

B. To change the HTTP method of the request automatically

C. To reuse data from one request in subsequent requests or tests

D. To generate random data for the request body

Solution

1. Step 1: Understand variable usage in Postman

   Variables store data that can be reused across requests and tests.

2. Step 2: Identify the role of response variables

   Setting variables from response allows using dynamic data from one request in others.

3. Final Answer:

   To reuse data from one request in subsequent requests or tests -> Option C

4. Quick Check:

   Variable reuse = Reuse data [OK]

Hint: Variables store response data for reuse in later requests [OK]

Common Mistakes:

• Thinking variables change HTTP methods
• Confusing variable setting with encryption
• Assuming variables generate random data

2. Which Postman script correctly sets an environment variable named token from a JSON response field auth.token?

easy

A. pm.variables.set('token', pm.response.auth.token);

B. pm.environment.set('token', pm.response.json().auth.token);

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

D. pm.globals.set('token', pm.response.body.auth.token);

Solution

1. Step 1: Identify correct method to set environment variable

   Use pm.environment.set(name, value) to set environment variables.

2. Step 2: Extract JSON response value correctly

   pm.response.json() parses JSON; access nested field with .auth.token.

3. Final Answer:

   pm.environment.set('token', pm.response.json().auth.token); -> Option B

4. Quick Check:

   Set environment variable = pm.environment.set [OK]

Hint: Use pm.environment.set with pm.response.json() for JSON fields [OK]

Common Mistakes:

• Using pm.environment.get instead of set
• Accessing response fields incorrectly
• Confusing pm.variables with environment variables

3. Given this Postman test script, what will be the value of the environment variable userId after execution?

const jsonData = pm.response.json();
pm.environment.set('userId', jsonData.data[0].id);

Response body:

{"data": [{"id": 42, "name": "Alice"}, {"id": 43, "name": "Bob"}]}

medium

A. 42

B. 43

C. undefined

D. Error: jsonData.data is not iterable

Solution

1. Step 1: Parse JSON response and access first element

   jsonData.data[0].id accesses the first object's id, which is 42.

2. Step 2: Set environment variable userId to this value

   pm.environment.set stores 42 as the value of userId.

3. Final Answer:

   42 -> Option A

4. Quick Check:

   First data id = 42 [OK]

Hint: Index 0 of data array gives first id value [OK]

Common Mistakes:

• Choosing second element's id (43)
• Assuming undefined due to wrong access
• Expecting runtime error incorrectly

4. You wrote this Postman test script to set a global variable from a response header:

pm.globals.set('sessionId', pm.response.headers.get('Session-ID'));

But the variable is not set after the request. What is the most likely reason?

medium

A. The header name is case-sensitive and should be 'session-id'

B. You must parse the response body before setting variables

C. pm.globals.set cannot set variables from headers

D. pm.response.headers.get() returns null if header is missing or name is wrong

Solution

1. Step 1: Understand header retrieval in Postman

   Header names are case-insensitive, but if the header is missing or name is wrong, get() returns null.

2. Step 2: Check why variable is not set

   If pm.response.headers.get('Session-ID') returns null, the variable is set to null or empty, appearing unset.

3. Final Answer:

   pm.response.headers.get() returns null if header is missing or name is wrong -> Option D

4. Quick Check:

   Header get returns null if missing [OK]

Hint: Check header name and existence before setting variable [OK]

Common Mistakes:

• Assuming header names are case-sensitive
• Believing pm.globals.set can't set from headers
• Forgetting to check if header exists

5. You want to set a collection variable authToken from a nested JSON response where the token may sometimes be missing. Which script correctly sets authToken to the token value if present, or to an empty string if missing?

hard

A. const token = pm.response.json()?.auth?.token ?? ''; pm.collectionVariables.set('authToken', token);

B. pm.collectionVariables.set('authToken', pm.response.json().auth.token);

C. if(pm.response.json().auth.token) { pm.collectionVariables.set('authToken', pm.response.json().auth.token); }

D. pm.collectionVariables.set('authToken', pm.response.json().auth?.token || null);

Solution

1. Step 1: Handle optional chaining to avoid errors if token missing

   Using ?. safely accesses nested properties without error if missing.

2. Step 2: Use nullish coalescing ?? to set empty string if token is undefined or null

   This ensures authToken is never undefined, avoiding test failures.

3. Step 3: Set collection variable with the safe token value

   pm.collectionVariables.set('authToken', token); stores the value correctly.

4. Final Answer:

   const token = pm.response.json()?.auth?.token ?? ''; pm.collectionVariables.set('authToken', token); -> Option A

5. Quick Check:

   Optional chaining + nullish coalescing = safe set [OK]

Hint: Use ?. and ?? to safely set variables from optional fields [OK]

Common Mistakes:

• Not handling missing token causing errors
• Setting variable without fallback value
• Using || which treats empty string as false