Response size in Postman - Build an Automation Script

Verify API response size is within expected limit

Preconditions (2)

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

Step 2: Check the size of the response body in bytes

Step 3: Verify that the response size is less than or equal to 5000 bytes

✅ Expected Result: The response size is less than or equal to 5000 bytes

Automation Requirements - Postman test scripts

Assertions Needed:

Response size is less than or equal to 5000 bytes

Best Practices:

Use pm.response.to.have.property('responseSize') to get response size

Use pm.expect for assertions

Write clear and simple test scripts

Avoid hardcoding values except for limits

Add comments for clarity

Automated Solution

Postman

pm.test('Response size is within 5000 bytes', function () {
    const responseSize = pm.response.responseSize();
    pm.expect(responseSize).to.be.at.most(5000);
});

This Postman test script checks the size of the response body.

First, it gets the response size in bytes using pm.response.responseSize().

Then, it asserts that the size is at most 5000 bytes using pm.expect.

This ensures the API response is not too large, which helps with performance and bandwidth.

Common Mistakes - 3 Pitfalls

Using pm.response.size instead of pm.response.responseSize()

Hardcoding the response size value inside the test without explanation

Not using pm.expect for assertions

Bonus Challenge

Now add data-driven testing with 3 different API endpoints and verify their response sizes are within limits

Show Hint

Practice

1. What does pm.response.size().body return in Postman?

easy

A. The total number of headers in the response

B. The HTTP status code of the response

C. The size of the response body in bytes

D. The time taken to receive the response

Solution

1. Step 1: Understand the method pm.response.size()

   This method returns an object containing sizes of different parts of the response.

2. Step 2: Identify what .body accesses

   The .body property specifically returns the size of the response body in bytes.

3. Final Answer:

   The size of the response body in bytes -> Option C

4. Quick Check:

   pm.response.size().body = response body size [OK]

Hint: Remember .body gives size in bytes of response content [OK]

Common Mistakes:

• Confusing response size with status code
• Thinking it returns header count
• Assuming it returns response time

2. Which of the following is the correct syntax to assert that the response body size is less than 5000 bytes in Postman test script?

easy

A. pm.response.size().body < 5000;

B. pm.expect(pm.response.size().body).to.be.below(5000);

C. assert(pm.response.body.size < 5000);

D. pm.response.size.body < 5000;

Solution

1. Step 1: Identify correct assertion syntax in Postman

   Postman uses Chai assertion library with pm.expect() for assertions.

2. Step 2: Check the correct property and method usage

   The correct way to get response body size is pm.response.size().body, and to assert it is below 5000 bytes, use to.be.below(5000).

3. Final Answer:

   pm.expect(pm.response.size().body).to.be.below(5000); -> Option B

4. Quick Check:

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

Hint: Use pm.expect() with to.be.below() for size assertions [OK]

Common Mistakes:

• Using incorrect property like pm.response.body.size
• Missing pm.expect() wrapper
• Using plain comparison without assertion

3. Given the following Postman test script:
<pre>pm.test('Body size check', () => { const size = pm.response.size().body; pm.expect(size).to.be.above(1000); pm.expect(size).to.be.below(3000); });
What will happen if the response body size is 3500 bytes?

medium

A. The test will fail because 3500 is not below 3000

B. The test will pass because 3500 is above 1000

C. The test will fail because 3500 is not above 1000

D. The test will pass because both conditions are true

Solution

1. Step 1: Analyze the assertions in the test

   There are two assertions: size must be above 1000 and below 3000 bytes.

2. Step 2: Check the given size against assertions

   3500 is above 1000 (true) but not below 3000 (false), so the second assertion fails.

3. Final Answer:

   The test will fail because 3500 is not below 3000 -> Option A

4. Quick Check:

   3500 < 3000? No, test fails [OK]

Hint: Check all assertions; one fail means test fails [OK]

Common Mistakes:

• Ignoring the upper bound assertion
• Assuming test passes if one assertion passes
• Confusing above and below conditions

4. You wrote this Postman test:

pm.test('Check response size', () => {
  pm.expect(pm.response.size.body).to.be.below(2000);
});

But the test always fails with an error. What is the problem?

medium

A. Incorrect property access; should use pm.response.size().body

B. The assertion method to.be.below() is invalid

C. pm.expect() cannot be used inside pm.test()

D. Response size cannot be checked in Postman tests

Solution

1. Step 1: Identify the property access error

   The code uses pm.response.size.body which is incorrect because size is a function, not an object.

2. Step 2: Correct the syntax

   The correct syntax is pm.response.size().body to get the response body size.

3. Final Answer:

   Incorrect property access; should use pm.response.size().body -> Option A

4. Quick Check:

   Use parentheses for size() function [OK]

Hint: Remember size() is a function, not a property [OK]

Common Mistakes:

• Missing parentheses after size()
• Misusing assertion methods
• Thinking pm.expect() is invalid inside pm.test()

5. You want to write a Postman test that fails if the response body size is either too small (less than 100 bytes) or too large (more than 5000 bytes). Which test script correctly implements this?

hard

A. pm.test('Body size limits', () => {
const size = pm.response.size().body;
pm.expect(size).to.be.within(100, 5000);
});

B. pm.test('Body size limits', () => {
const size = pm.response.size().body;
pm.expect(size < 100 || size > 5000).to.be.false;
});

C. pm.test('Body size limits', () => {
const size = pm.response.size().body;
pm.expect(size).to.be.above(100) && pm.expect(size).to.be.below(5000);
});

D. pm.test('Body size limits', () => {
const size = pm.response.size().body;
if(size < 100 || size > 5000) {
throw new Error('Body size out of limits');
}
});

Solution

1. Step 1: Understand the requirement

   The test should fail if size is less than 100 or greater than 5000 bytes.

2. Step 2: Evaluate each option's logic

   pm.test('Body size limits', () => { const size = pm.response.size().body; pm.expect(size).to.be.within(100, 5000); }); uses within(100, 5000) which includes 100 and 5000, but does not fail outside range explicitly.
   pm.test('Body size limits', () => { const size = pm.response.size().body; pm.expect(size < 100 || size > 5000).to.be.false; }); uses a boolean expression inside pm.expect() which is not a valid assertion.
   pm.test('Body size limits', () => { const size = pm.response.size().body; pm.expect(size).to.be.above(100) && pm.expect(size).to.be.below(5000); }); uses two assertions combined with && which does not work as expected in Postman.
   pm.test('Body size limits', () => { const size = pm.response.size().body; if(size < 100 || size > 5000) { throw new Error('Body size out of limits'); } }); throws an error explicitly if size is out of limits, causing the test to fail correctly.

3. Final Answer:

   Option D correctly fails the test when size is out of limits -> Option D

4. Quick Check:

   Throw error to fail test on condition [OK]

Hint: Use throw Error to fail test on size limits [OK]

Common Mistakes:

• Using invalid assertion syntax for range checks
• Combining assertions incorrectly with &&
• Not explicitly failing test on condition