Why pre-request scripts prepare data in Postman - The Real Reasons

The Big Idea

What if your tests could always have the right data without you lifting a finger?

The Scenario

Imagine you need to test an API that requires a fresh token or dynamic data every time you send a request. You open Postman and manually update the token or data before each request.

The Problem

This manual updating is slow and easy to forget. You might send requests with expired tokens or wrong data, causing test failures and confusion. It wastes time and makes tests unreliable.

The Solution

Pre-request scripts automatically prepare and set the needed data before each request runs. This means tokens, timestamps, or any dynamic info are always fresh and correct without manual work.

Before vs After

✗ Before

Set token manually in headers before each request

✓ After

Use pre-request script to fetch and set token automatically

What It Enables

It enables fully automated, reliable API tests that always use the right data at the right time.

Real Life Example

When testing a login API, a pre-request script can get a new access token automatically so you never send expired tokens.

Key Takeaways

Manual data updates are slow and error-prone.

Pre-request scripts automate data preparation before requests.

This makes API testing faster, more reliable, and less stressful.

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