Using Chai assertion library in Postman - Build an Automation Script

Verify API response status and body using Chai assertions in Postman

Preconditions (2)

Step 1: Send a GET request to https://jsonplaceholder.typicode.com/posts/1

Step 2: In the Tests tab, write Chai assertions to verify the response status is 200

Step 3: Verify the response body contains userId equal to 1

Step 4: Verify the response body has a title property that is a non-empty string

✅ Expected Result: Test script runs successfully with all assertions passing, confirming status 200 and correct response body properties

Automation Requirements - Postman test scripts with Chai assertion library

Assertions Needed:

response status is 200

response body userId equals 1

response body title is a non-empty string

Best Practices:

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

Parse response JSON once and reuse it

Use descriptive assertion messages

Avoid hardcoding values except for test verification

Automated Solution

Postman

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

const responseJson = pm.response.json();

pm.test('userId is 1', () => {
    pm.expect(responseJson.userId, 'userId should be 1').to.eql(1);
});

pm.test('title is a non-empty string', () => {
    pm.expect(responseJson.title, 'title should be a non-empty string').to.be.a('string').and.not.empty;
});

The first test checks the HTTP status code is exactly 200 using pm.response.to.have.status(200). This confirms the request succeeded.

Next, we parse the response JSON once with pm.response.json() and store it in responseJson to avoid repeated parsing.

The second test asserts that the userId property equals 1 using pm.expect().to.eql() with a clear message.

The third test verifies the title property is a string and not empty using chained Chai assertions .to.be.a('string').and.not.empty. This ensures the title is valid text.

Each assertion has a descriptive message to help understand failures. This script follows Postman and Chai best practices for clarity and efficiency.

Common Mistakes - 3 Pitfalls

Not parsing response JSON before accessing properties

{'mistake': 'Using pm.expect(pm.response.code).to.equal(200) instead of pm.response.to.have.status(200)', 'why_bad': "Less readable and does not use Postman's built-in status assertion which gives better error messages", 'correct_approach': 'Use pm.response.to.have.status(200) for status code assertions'}

Hardcoding assertion values without descriptive messages

Bonus Challenge

Now add data-driven testing with 3 different post IDs (1, 2, 3) verifying the same assertions for each

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