Response time assertions in Postman - Build an Automation Script

Verify API response time is under 500 milliseconds

Preconditions (2)

Step 1: Open Postman and create a new GET request

Step 2: Enter the API endpoint URL in the request URL field

Step 3: Send the request by clicking the Send button

Step 4: Observe the response time shown in Postman

Step 5: Verify that the response time is less than 500 milliseconds

✅ Expected Result: The API responds successfully and the response time is less than 500 milliseconds

Automation Requirements - Postman Tests (JavaScript)

Assertions Needed:

Response status code is 200

Response time is less than 500 milliseconds

Best Practices:

Use pm.response.to.have.status for status code assertion

Use pm.expect(pm.response.responseTime).to.be.below(500) for response time assertion

Write clear and simple test scripts inside the Tests tab

Avoid hardcoding values except for thresholds like 500 ms

Automated Solution

Postman

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

pm.test('Response time is less than 500 ms', () => {
    pm.expect(pm.response.responseTime).to.be.below(500);
});

The first test checks that the API returns a status code 200, which means success.

The second test checks that the response time is below 500 milliseconds using pm.response.responseTime.

Both tests use Postman's built-in pm object and chai assertion library for clear and readable assertions.

This script should be placed in the Tests tab of the Postman request.

Common Mistakes - 3 Pitfalls

Checking response time with incorrect property like pm.response.time

Using hardcoded status codes without assertion methods

{'mistake': 'Not placing the test script inside the Tests tab', 'why_bad': "Scripts outside the Tests tab won't run after the request, so no assertions happen.", 'correct_approach': 'Always write test scripts inside the Tests tab of the Postman request.'}

Bonus Challenge

Now add data-driven testing with 3 different API endpoints and verify each response time is under 500 ms

Show Hint

Practice

1. What does pm.response.responseTime represent in Postman tests?

easy

A. The size of the API response in bytes

B. The number of API requests sent

C. The HTTP status code of the response

D. The time taken by the API to respond in milliseconds

Solution

1. Step 1: Understand the property pm.response.responseTime

   This property in Postman returns the time taken by the API server to send a response, measured in milliseconds.

2. Step 2: Differentiate from other response properties

   It does not represent response size, status code, or request count, which are different properties.

3. Final Answer:

   The time taken by the API to respond in milliseconds -> Option D

4. Quick Check:

   Response time = API speed [OK]

Hint: Response time means how fast API replies in ms [OK]

Common Mistakes:

• Confusing response time with response size
• Mixing response time with HTTP status code
• Thinking it counts number of requests

2. Which of the following is the correct syntax to assert that response time is less than 500 milliseconds in Postman test script?

easy

A. pm.test('Response time is less than 500ms', () => { pm.expect(pm.response.time).to.be.above(500); });

B. pm.test('Response time is less than 500ms', () => { pm.expect(pm.response.responseTime).to.be.below(500); });

C. pm.test('Response time is less than 500ms', () => { pm.expect(responseTime).to.equal(500); });

D. pm.test('Response time is less than 500ms', () => { pm.expect(pm.response.responseTime).to.be.above(500); });

Solution

1. Step 1: Identify correct property and assertion method

   The correct property for response time is pm.response.responseTime. To check if it is less than 500ms, use to.be.below(500).

2. Step 2: Verify syntax correctness

   pm.test('Response time is less than 500ms', () => { pm.expect(pm.response.responseTime).to.be.below(500); }); uses the correct property and assertion syntax. Other options use wrong properties or wrong comparison methods.

3. Final Answer:

   pm.test('Response time is less than 500ms', () => { pm.expect(pm.response.responseTime).to.be.below(500); }); -> Option B

4. Quick Check:

   Use pm.response.responseTime with to.be.below() [OK]

Hint: Use pm.response.responseTime with to.be.below() for less than checks [OK]

Common Mistakes:

• Using wrong property like pm.response.time
• Using to.be.above() instead of to.be.below()
• Missing pm.expect wrapper

3. Given the following Postman test code, what will be the test result if the API response time is 450 ms?

pm.test('Response time is acceptable', () => {
  pm.expect(pm.response.responseTime).to.be.below(400);
});

medium

A. Test will pass because 450 is below 400

B. Test will pass because 450 equals 400

C. Test will fail because 450 is not below 400

D. Test will error due to syntax mistake

Solution

1. Step 1: Understand the assertion condition

   The test expects pm.response.responseTime to be below 400 milliseconds.

2. Step 2: Compare actual response time with condition

   The actual response time is 450 ms, which is greater than 400 ms, so the condition fails.

3. Final Answer:

   Test will fail because 450 is not below 400 -> Option C

4. Quick Check:

   450 > 400 means test fails [OK]

Hint: Check if actual time is less than threshold to pass [OK]

Common Mistakes:

• Assuming 450 is below 400
• Confusing pass/fail logic
• Ignoring comparison operator meaning

4. Identify the error in this Postman test script for response time assertion:

pm.test('Response time check', function() {
  pm.expect(pm.response.responseTime).to.be.lessThan(300);
});

medium

A. The assertion method lessThan is incorrect in Postman tests

B. The function syntax is invalid

C. The property pm.response.responseTime does not exist

D. The test name is missing

Solution

1. Step 1: Check assertion method correctness

   Postman uses Chai assertion library where the correct method to check less than is to.be.below(), not lessThan().

2. Step 2: Verify other parts of the script

   The function syntax is valid, the property exists, and the test name is present.

3. Final Answer:

   The assertion method lessThan is incorrect in Postman tests -> Option A

4. Quick Check:

   Use to.be.below() not lessThan() [OK]

Hint: Use to.be.below() for less than, not lessThan() [OK]

Common Mistakes:

• Using lessThan() instead of to.be.below()
• Thinking lessThan() is valid Chai syntax
• Ignoring assertion library conventions

5. You want to write a Postman test that fails if the response time is more than 1000 ms but passes if it is exactly 1000 ms or less. Which assertion code correctly implements this logic?

hard

A. pm.test('Response time check', () => { pm.expect(pm.response.responseTime).to.be.at.most(1000); });

B. pm.test('Response time check', () => { pm.expect(pm.response.responseTime).to.be.below(1000); });

C. pm.test('Response time check', () => { pm.expect(pm.response.responseTime).to.equal(1000); });

D. pm.test('Response time check', () => { pm.expect(pm.response.responseTime).to.be.above(1000); });

Solution

1. Step 1: Understand the requirement

   The test should pass if response time is 1000 ms or less, and fail if more than 1000 ms.

2. Step 2: Choose correct assertion method

   to.be.at.most(1000) checks if value is less than or equal to 1000, matching the requirement. to.be.below(1000) excludes 1000, to.equal(1000) only passes exactly 1000, and to.be.above(1000) is opposite.

3. Final Answer:

   pm.test('Response time check', () => { pm.expect(pm.response.responseTime).to.be.at.most(1000); }); -> Option A

4. Quick Check:

   Use to.be.at.most() for <= checks [OK]

Hint: Use to.be.at.most() for less than or equal assertions [OK]

Common Mistakes:

• Using to.be.below() excludes equal value
• Using to.equal() only matches exact value
• Using to.be.above() reverses logic