Dynamic assertion values in Postman

Introduction

Dynamic assertion values let you check test results that change each time. This helps make tests flexible and real.

When testing APIs that return timestamps or unique IDs.

When the response data changes with each request, like random quotes or user sessions.

When you want to compare a value to a previous response or environment variable.

When validating data that depends on current date or time.

When you need to check if a value exists or matches a pattern instead of exact text.

Syntax

Postman

pm.test("Test name", function () {
    let jsonData = pm.response.json();
    pm.expect(jsonData.key).to.eql(pm.environment.get("variableName"));
});

You use pm.environment.get() or pm.variables.get() to get dynamic values.

Assertions compare response data to these dynamic values instead of fixed ones.

Examples

This test checks if the userId in the response matches the userId saved in environment variables.

Postman

pm.test("Check user ID matches stored ID", function () {
    let jsonData = pm.response.json();
    pm.expect(jsonData.userId).to.eql(pm.environment.get("userId"));
});

This test compares the date in the response to today's date dynamically.

Postman

pm.test("Response has current date", function () {
    let jsonData = pm.response.json();
    let today = new Date().toISOString().slice(0, 10);
    pm.expect(jsonData.date).to.eql(today);
});

This test checks that the token field exists and is not empty, useful when token changes every time.

Postman

pm.test("Token exists and is not empty", function () {
    let jsonData = pm.response.json();
    pm.expect(jsonData.token).to.be.a('string').and.not.empty;
});

Sample Program

This script sets a dynamic expected user ID in environment variables, then tests if the API response userId matches it.

Postman

pm.environment.set("expectedUserId", "12345");

pm.test("User ID matches expected dynamic value", function () {
    let jsonData = pm.response.json();
    pm.expect(jsonData.userId).to.eql(pm.environment.get("expectedUserId"));
});

Important Notes

Always set or update dynamic values before running assertions.

Use environment or collection variables to store dynamic data between requests.

Dynamic assertions help tests stay valid even when data changes often.

Summary

Dynamic assertion values make tests flexible and realistic.

Use Postman variables to store and compare changing data.

Helps test APIs that return different data each time.

Practice

1. What is the main purpose of using dynamic assertion values in Postman tests?

easy

A. To make tests flexible by comparing values that change each time

B. To hardcode fixed values for all test runs

C. To disable assertions temporarily

D. To speed up test execution by skipping checks

Solution

1. Step 1: Understand dynamic assertion values

   Dynamic assertion values allow tests to compare values that can change between API calls, making tests adaptable.

2. Step 2: Identify the purpose in Postman

   Using dynamic values helps verify API responses that return different data each time, ensuring tests remain valid.

3. Final Answer:

   To make tests flexible by comparing values that change each time -> Option A

4. Quick Check:

   Dynamic assertions = flexible tests [OK]

Hint: Dynamic values adapt tests to changing data [OK]

Common Mistakes:

• Thinking dynamic means disabling assertions
• Confusing dynamic with fixed values
• Assuming dynamic values speed up tests

2. Which of the following is the correct way to use a Postman variable in an assertion to check a response value dynamically?

easy

A. pm.expect(pm.response.json().data.id).to.eql('{{$randomId}}');

B. pm.expect(pm.response.json().data.id).to.eql(pm.variables.get('randomId'));

C. pm.expect(pm.response.json().data.id).to.eql('$randomId');

D. pm.expect(pm.response.json().data.id).to.eql(randomId);

Solution

1. Step 1: Recall Postman variable syntax in scripts

   In test scripts, variables are accessed using pm.variables.get('variableName').

2. Step 2: Identify correct assertion usage

   pm.expect(pm.response.json().data.id).to.eql(pm.variables.get('randomId')); correctly uses pm.variables.get('randomId') to get the variable value for comparison.

3. Final Answer:

   pm.expect(pm.response.json().data.id).to.eql(pm.variables.get('randomId')); -> Option B

4. Quick Check:

   Use pm.variables.get() for dynamic values [OK]

Hint: Use pm.variables.get('name') to access variables in tests [OK]

Common Mistakes:

• Using {{}} inside test scripts instead of pm.variables.get()
• Using variable name as plain string without get()
• Assuming $variable syntax works in scripts

3. Given this Postman test snippet:

pm.test('Check user ID', () => {
  const expectedId = pm.variables.get('userId');
  pm.expect(pm.response.json().id).to.eql(expectedId);
});

If the variable userId is set to 123 and the response JSON is {"id": 124}, what will be the test result?

medium

A. Test passes because values are close

B. Test throws syntax error

C. Test fails because 124 != 123

D. Test is skipped due to missing variable

Solution

1. Step 1: Compare expected and actual values

   The expected value from variable userId is 123, but response JSON has id 124.

2. Step 2: Evaluate assertion outcome

   The assertion to.eql(123) vs actual 124 will fail because they are not equal.

3. Final Answer:

   Test fails because 124 != 123 -> Option C

4. Quick Check:

   Mismatch values cause test failure [OK]

Hint: Check exact values in assertions to predict pass/fail [OK]

Common Mistakes:

• Assuming close numbers pass equality
• Confusing syntax error with assertion failure
• Ignoring variable value in test

4. You wrote this Postman test:

pm.test('Check token', () => {
  const token = pm.variables.get('authToken');
  pm.expect(pm.response.json().token).to.eql(token);
});

But the test always fails even though the token looks correct. What is the most likely cause?

medium

A. The variable authToken is not set or is empty

B. The response JSON does not contain a token field

C. The assertion syntax is invalid

D. Postman does not support dynamic assertions

Solution

1. Step 1: Check variable usage

   If authToken is missing or empty, pm.variables.get('authToken') returns undefined or empty string.

2. Step 2: Understand assertion failure

   Comparing response token to undefined or empty string causes assertion to fail even if response token is correct.

3. Final Answer:

   The variable authToken is not set or is empty -> Option A

4. Quick Check:

   Unset variables cause assertion mismatches [OK]

Hint: Always verify variables are set before assertions [OK]

Common Mistakes:

• Assuming syntax error without checking variables
• Ignoring missing response fields
• Believing Postman lacks dynamic assertion support

5. You want to test an API that returns a new order ID each time. You store the previous order ID in a variable prevOrderId. Which Postman test code correctly asserts that the new order ID is different from the previous one?

hard

A. pm.expect(pm.response.json().orderId).to.eql(pm.variables.get('prevOrderId'));

B. pm.expect(pm.response.json().orderId).to.be.lessThan(pm.variables.get('prevOrderId'));

C. pm.expect(pm.response.json().orderId).to.be.greaterThan(pm.variables.get('prevOrderId'));

D. pm.expect(pm.response.json().orderId).not.eql(pm.variables.get('prevOrderId'));

Solution

1. Step 1: Understand the requirement

   The test should confirm the new order ID is different from the previous one, so equality must fail.

2. Step 2: Choose correct assertion

   pm.expect(pm.response.json().orderId).not.eql(pm.variables.get('prevOrderId')); uses not.eql to assert the new ID is not equal to the previous ID, matching the requirement.

3. Final Answer:

   pm.expect(pm.response.json().orderId).not.eql(pm.variables.get('prevOrderId')); -> Option D

4. Quick Check:

   Use not.eql to assert values differ [OK]

Hint: Use not.eql to check values are different [OK]

Common Mistakes:

• Using eql to check difference (wrong)
• Assuming greaterThan always works for IDs
• Confusing lessThan with difference check