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

Show Hint

Practice

1. What is the main purpose of using the Chai assertion library in Postman tests?

easy

A. To create user interface elements

B. To write clear and readable checks for API responses

C. To send HTTP requests faster

D. To store environment variables

Solution

1. Step 1: Understand Chai's role in testing

   Chai is used to write assertions that check if API responses meet expectations.

2. Step 2: Identify the correct purpose in Postman context

   In Postman, Chai helps create readable tests that verify API behavior.

3. Final Answer:

   To write clear and readable checks for API responses -> Option B

4. Quick Check:

   Chai assertions = readable API checks [OK]

Hint: Chai is for checking API results clearly [OK]

Common Mistakes:

• Confusing Chai with request sending
• Thinking Chai creates UI elements
• Mixing Chai with environment variable storage

2. Which of the following is the correct syntax to assert that the response status code is 200 using Chai in Postman?

easy

A. pm.expect(response.code).to.equal(200);

B. pm.expect(pm.response.status).to.equal(200);

C. pm.expect(pm.response.code).to.equal(200);

D. pm.expect(response.status).to.be(200);

Solution

1. Step 1: Identify the correct Postman response object

   The response object in Postman is accessed via pm.response.

2. Step 2: Use Chai syntax to check status code

   The correct Chai assertion is pm.expect(pm.response.code).to.equal(200);.

3. Final Answer:

   pm.expect(pm.response.code).to.equal(200); -> Option C

4. Quick Check:

   Status code check uses pm.response.code [OK]

Hint: Use pm.response.code with to.equal for status [OK]

Common Mistakes:

• Using wrong object like response.code
• Using to.be instead of to.equal
• Missing pm.response prefix

3. Given this Postman test code:

pm.test('Check response body', () => {
  pm.expect(pm.response.text()).to.include('success');
});

What will happen if the response body is 'Operation was successful'?

medium

A. Test will pass only if response code is 200

B. Test will fail because 'success' is not exactly matched

C. Test will throw a syntax error

D. Test will pass because 'success' is included in the response

Solution

1. Step 1: Understand the assertion used

   The assertion checks if the response text includes the substring 'success'.

2. Step 2: Check if 'success' is in 'Operation was successful'

   The word 'successful' contains 'success' as a substring, so the assertion passes.

3. Final Answer:

   Test will pass because 'success' is included in the response -> Option D

4. Quick Check:

   Substring check includes 'success' [OK]

Hint: Include checks pass if substring exists anywhere [OK]

Common Mistakes:

• Expecting exact match instead of substring
• Confusing syntax error with assertion failure
• Assuming status code affects this test

4. Identify the error in this Postman test code snippet:

pm.test('Status is 404', () => {
  pm.expect(pm.response.status).to.equal(404);
});

medium

A. pm.response.status is not the correct property for status code

B. to.equal should be to.be.equal

C. pm.test should be pm.expect

D. Missing semicolon after pm.test

Solution

1. Step 1: Check the property used for status code

   The correct property for status code in Postman is pm.response.code, not pm.response.status.

2. Step 2: Confirm Chai syntax correctness

   The to.equal syntax is correct and pm.test is used properly.

3. Final Answer:

   pm.response.status is not the correct property for status code -> Option A

4. Quick Check:

   Status code property is pm.response.code [OK]

Hint: Use pm.response.code for status, not pm.response.status [OK]

Common Mistakes:

• Using pm.response.status instead of pm.response.code
• Adding unnecessary 'to.be' in assertion
• Confusing pm.test with pm.expect

5. You want to write a Postman test that asserts the JSON response has a property userId with a value greater than 100. Which code snippet correctly uses Chai assertions to do this?

hard

A. pm.test('userId > 100', () => {
const jsonData = pm.response.json();
pm.expect(jsonData.userId).to.be.above(100);
});

B. pm.test('userId > 100', () => {
const jsonData = pm.response.json();
pm.expect(jsonData.userId).to.equal(100);
});

C. pm.test('userId > 100', () => {
pm.expect(pm.response.text()).to.include('userId > 100');
});

D. pm.test('userId > 100', () => {
pm.expect(pm.response.code).to.be.above(100);
});

Solution

1. Step 1: Parse JSON response correctly

   Use pm.response.json() to get the response as an object.

2. Step 2: Use Chai's 'above' assertion on userId property

   Check that jsonData.userId is greater than 100 with to.be.above(100).

3. Final Answer:

   pm.test('userId > 100', () => { const jsonData = pm.response.json(); pm.expect(jsonData.userId).to.be.above(100); }); -> Option A

4. Quick Check:

   Parse JSON then assert property above value [OK]

Hint: Parse JSON then use to.be.above for numeric checks [OK]

Common Mistakes:

• Using to.equal instead of to.be.above
• Checking response text instead of JSON property
• Checking status code instead of userId