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

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