Why pre-request scripts prepare data in Postman - Automation Benefits in Action

Verify pre-request script prepares data correctly before API call

Preconditions (3)

Step 1: Open the Postman collection and select the API request

Step 2: Go to the Pre-request Script tab

Step 3: Write a script that generates a token and saves it to an environment variable 'authToken'

Step 4: Go to the Headers tab and add 'Authorization' header with value 'Bearer {{authToken}}'

Step 5: Send the API request

Step 6: Observe the request headers and response

✅ Expected Result: The API request includes the Authorization header with the token generated by the pre-request script, and the API responds successfully using the prepared data

Automation Requirements - Postman test scripts

Assertions Needed:

Verify environment variable 'authToken' is set before the request

Verify the Authorization header contains the correct token

Verify the API response status code is 200

Best Practices:

Use pm.environment.set() to store data in pre-request scripts

Use pm.request.headers to verify headers

Use pm.test() for assertions

Keep scripts simple and focused on preparing data

Automated Solution

Postman

pm.test('Pre-request script sets authToken', function () {
    pm.expect(pm.environment.get('authToken')).to.be.a('string').and.not.empty;
});

pm.test('Authorization header contains authToken', function () {
    const authHeader = pm.request.headers.get('Authorization');
    const token = pm.environment.get('authToken');
    pm.expect(authHeader).to.equal(`Bearer ${token}`);
});

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

The first test checks that the pre-request script has set the environment variable authToken to a non-empty string before the request is sent.

The second test verifies that the Authorization header in the request includes the token prepared by the pre-request script, ensuring the API receives the correct data.

The third test confirms the API responded with status code 200, indicating success.

Using pm.environment.set() in the pre-request script stores data for use in the request. The tests use pm.test() and pm.expect() for clear, readable assertions.

Common Mistakes - 3 Pitfalls

{'mistake': 'Not setting environment variable in pre-request script', 'why_bad': "The token or data won't be available for the request, causing failures.", 'correct_approach': 'Always use pm.environment.set() or pm.variables.set() to store data before the request.'}

Hardcoding token value instead of generating dynamically

Not verifying that the header uses the prepared token

Bonus Challenge

Now add data-driven testing by preparing different tokens for three different user roles and verify the API response for each.

Show Hint

Practice

1. Why do pre-request scripts run before sending a request in Postman?

easy

A. To display the test results

B. To check the response status code

C. To prepare or update data needed for the request

D. To log the response time

Solution

1. Step 1: Understand the role of pre-request scripts

   Pre-request scripts run before the request is sent to prepare or update any data needed for the request.

2. Step 2: Differentiate from other script types

   Scripts that check response status or display results run after the request, not before.

3. Final Answer:

   To prepare or update data needed for the request -> Option C

4. Quick Check:

   Pre-request scripts prepare data = C [OK]

Hint: Pre-request scripts run before requests to set data [OK]

Common Mistakes:

• Confusing pre-request scripts with test scripts
• Thinking pre-request scripts check responses
• Assuming pre-request scripts run after requests

2. Which of the following is the correct way to set a variable named token in a pre-request script?

easy

A. pm.variables.set('token', 'abc123');

B. pm.environment.get('token', 'abc123');

C. pm.variables.get('token', 'abc123');

D. pm.environment.delete('token', 'abc123');

Solution

1. Step 1: Identify the correct method to set a variable

   In Postman scripts, pm.variables.set or pm.environment.set is used to set variables. Here, pm.variables.set('token', 'abc123'); correctly sets the variable.

2. Step 2: Understand other methods

   pm.environment.get and pm.variables.get retrieve variables, not set them. pm.environment.delete removes variables.

3. Final Answer:

   pm.variables.set('token', 'abc123'); -> Option A

4. Quick Check:

   Set variable uses .set() method = B [OK]

Hint: Use .set() to assign variables in pre-request scripts [OK]

Common Mistakes:

• Using .get() instead of .set() to assign variables
• Confusing environment and variables methods
• Trying to delete variables when setting them

3. What will be the value of the variable timestamp after running this pre-request script?

const now = new Date().getTime();
pm.variables.set('timestamp', now);

medium

A. A number representing milliseconds since Jan 1, 1970

B. An error because pm.variables.set is incorrect

C. Undefined because now is not set

D. A string representing the current date

Solution

1. Step 1: Understand new Date().getTime()

   This returns the current time in milliseconds since January 1, 1970 (Unix epoch) as a number.

2. Step 2: Check variable assignment

   The script sets the variable timestamp to this number using pm.variables.set, which is correct syntax.

3. Final Answer:

   A number representing milliseconds since Jan 1, 1970 -> Option A

4. Quick Check:

   getTime() returns milliseconds number = D [OK]

Hint: getTime() returns milliseconds number, not string [OK]

Common Mistakes:

• Thinking getTime() returns a date string
• Assuming variable is undefined
• Confusing set() method usage

4. Identify the error in this pre-request script:

pm.environment.set('authToken', getToken());
function getToken() {
  return getToken();
}

medium

A. The function getToken is missing parentheses

B. pm.environment.set is not a valid method

C. Variables cannot be set in pre-request scripts

D. The function getToken causes infinite recursion

Solution

1. Step 1: Analyze the function call

   The script calls getToken() inside pm.environment.set('authToken', getToken()). The function getToken() returns getToken(), which calls itself again.

2. Step 2: Identify recursion problem

   This causes infinite recursion because getToken() calls itself directly, leading to a stack overflow.

3. Final Answer:

   The function getToken causes infinite recursion -> Option D

4. Quick Check:

   Function calls itself recursively = infinite recursion [OK]

Hint: Avoid recursive functions in pre-request scripts [OK]

Common Mistakes:

• Thinking pm.environment.set is invalid
• Missing that getToken calls itself recursively
• Assuming variables can't be set in pre-request scripts

5. You want to generate a random user ID before each request and store it in an environment variable userId. Which pre-request script correctly achieves this?

hard

A. const id = Math.random() * 1000;
pm.environment.get('userId', id);

B. const id = Math.floor(Math.random() * 1000);
pm.environment.set('userId', id);

C. pm.environment.set('userId', Math.random());

D. const id = Math.floor(Math.random() * 1000);
pm.environment.delete('userId');

Solution

1. Step 1: Generate a random integer ID

   Math.floor(Math.random() * 1000) generates a random whole number between 0 and 999, suitable for a user ID.

2. Step 2: Correctly set the environment variable

   pm.environment.set('userId', id); correctly stores the generated ID in the environment variable userId.

3. Final Answer:

   const id = Math.floor(Math.random() * 1000); pm.environment.set('userId', id); -> Option B

4. Quick Check:

   Use Math.floor and .set() to store random ID = A [OK]

Hint: Use Math.floor and pm.environment.set to save random IDs [OK]

Common Mistakes:

• Using .get() instead of .set() to store variables
• Not rounding random numbers to integers
• Deleting variables instead of setting them