Writing Tests in Postman

# CHAPTER 13

Writing Tests in Postman #

1. Introduction #

2. Learning Objectives #

• Locate and use the "Tests" tab in the Request Builder.

• Understand the pm.test() and pm.expect() syntax.

• Write a basic test to verify an HTTP status code.

• Parse a JSON response body into a JavaScript object.

• Write assertions to validate specific data points in the JSON.

• Use Postman's built-in "Snippets" to generate test code quickly.

3. Beginner-Friendly Explanation #

In Postman, writing a test is creating that grading stencil. You write a small piece of code that says "The status code MUST be 200." When you hit Send, Postman fetches the data, applies the stencil, and instantly gives you a green PASS or a red FAIL.

4. Real-World Examples #

• Regression Testing: Before a company launches a new update, they press a single button. Postman runs 500 tests automatically. If one test fails (e.g., "Expected price to be an integer, but got a string"), the launch is stopped.

• Performance Thresholds: You write a test that says pm.expect(pm.response.responseTime).to.be.below(500). If the API takes longer than half a second to reply, the test fails, alerting developers to a performance bottleneck.

5. The Tests Tab and Snippets #

Step 1: Open a request (e.g., GET https://jsonplaceholder.typicode.com/users/1). Step 2: Click the Tests tab (next to Pre-request Script). Step 3: Look at the far right edge of the screen. You will see a list of blue links under the title Snippets. Step 4: Click the snippet titled "Status code: Code is 200". Postman instantly types this code for you:

pm.test("Status code is 200", function () {
    pm.response.to.have.status(200);
});

6. Writing Data Validation Tests #

Step 1: Parse the Response Before we can test the data, we must convert the raw text response into a JavaScript object.

// This line extracts the JSON body
const jsonData = pm.response.json();

Step 2: Write the Assertion Let's verify that the user's name is exactly "Leanne Graham".

pm.test("Verify User's Name", function () {
    // We expect the 'name' property of our json data to equal 'Leanne Graham'
    pm.expect(jsonData.name).to.eql("Leanne Graham");
});

7. Combining Multiple Tests #

// Test 1: Check Status
pm.test("Status is successful", function () {
    pm.response.to.have.status(200);
});

// Test 2: Check Speed
pm.test("Response is faster than 500ms", function () {
    pm.expect(pm.response.responseTime).to.be.below(500);
});

// Test 3: Check Data Structure
const jsonData = pm.response.json();
pm.test("User has an email address", function () {
    pm.expect(jsonData.email).to.be.a("string");
    // You can have multiple assertions in one test!
    pm.expect(jsonData.email.length).to.be.above(0);
});

8. Viewing Test Results #

1. 1. Click the Test Results tab.

1. 2. You will see a green PASS or a red FAIL next to each test name you defined (e.g., "Verify User's Name").

1. 3. If a test fails, Postman tells you exactly why: *AssertionError: expected 'Bob' to deeply equal 'Leanne Graham'*.

9. Best Practices #

• Use Descriptive Test Names: pm.test("Test 1") is terrible. pm.test("User object contains a valid email") is excellent. If a test fails, you want to know exactly what broke just by reading the title.

• Fail Gracefully: If you try to parse JSON but the server returns a 500 HTML error page, pm.response.json() will crash your script. It's often safer to verify the status is 200 *before* trying to parse complex JSON.

• Use Snippets: The snippets list on the right side of the screen contains the syntax for almost everything you need. Use them to learn!

10. Common Mistakes #

• Syntax Errors: Missing a closing parenthesis ) or bracket } in JavaScript will crash the test execution. Use the red squiggly lines in the editor to spot typos.

• Testing Dynamic Data as Static Data: Don't write pm.expect(jsonData.timestamp).to.eql("2023-10-15") if the API returns the current date. It will pass today and fail tomorrow. Instead, test the *type*: pm.expect(jsonData.timestamp).to.be.a("string").

11. Mini Exercises #

1. 1. Look at the Snippets menu. Find the snippet that checks if a string is present anywhere in the response body. What is the code?

1. 2. If jsonData.id equals 42, write a single line of pm.expect() code to verify it.

12. Coding/Testing Challenges #

1. 1. Verify that the userId is equal to 1.

1. 2. Verify that the title is of type "string".

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the process of validating a JSON response in Postman from start to finish.

• Q: What is an "Assertion" in the context of API testing? Give an example of syntax used in Postman (Chai library).

• Q: Why is automated testing via Postman scripts preferred over manual verification?

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #