Writing Tests and Assertions

Postman is a versatile tool not just for API testing but also for writing tests and assertions to ensure that your APIs work as expected. In this tutorial, we’ll explore Postman’s testing capabilities, including pre-request and test scripts, and how to write assertions to validate API responses.

Introduction

Writing tests and assertions is a crucial part of API testing. Postman simplifies this process by providing a robust scripting environment for both pre-request and test scripts. These scripts allow you to automate and validate various aspects of your API requests and responses.

Pre-request Scripts

Pre-request scripts are executed before sending a request. They are useful for setting up data or authentication before the request is made. Let’s look at an example:

Create a new request and go to the “Pre-request Script” tab.
In the script editor, you can write JavaScript code to set variables, headers, or perform any necessary actions before the request is sent.

// Example pre-request script to set an authentication header

pm.request.headers.add({

key: 'Authorization',

value: 'Bearer ' + pm.environment.get('access_token')

});

When you send the request, the pre-request script is executed, and the request is sent with the added header.

Writing Test Scripts

Test scripts are executed after the response is received. You can use test scripts to validate the response, extract data, and perform various checks. Here’s an example:

In the same request, go to the “Tests” tab.
In the test script editor, you can write JavaScript code to check the response, extract data, and perform assertions.

// Example test script to check the status code and response structure

pm.test("Status code is 200", function () {

pm.response.to.have.status(200);

});

pm.test("Response has expected fields", function () {

var jsonData = pm.response.json();

pm.expect(jsonData).to.have.property('id');

pm.expect(jsonData).to.have.property('name');

});

When you send the request, the test script is executed, and the assertions are checked.

Writing Assertions

Assertions are checks that verify whether the response meets your expectations. Postman provides a simple and expressive syntax for writing assertions. Here are a few common assertions:

pm.response.to.have.status(code): Checks if the response has the expected HTTP status code.
pm.expect(value).to.equal(expected): Checks if value is equal to the expected value.
pm.expect(value).to.be.a('type'): Checks if value is of the specified data type.
pm.expect(value).to.have.property('property'): Checks if an object has the specified property.
pm.expect(value).to.have.length(n): Checks if an array or string has the specified length.

Conclusion

In this tutorial, we’ve explored the testing capabilities of Postman, including pre-request and test scripts, and how to write assertions to validate API responses. Postman’s scripting environment is powerful, allowing you to automate various aspects of API testing and ensure that your APIs work as expected.

By mastering these testing features, you can create comprehensive test suites to validate your APIs thoroughly. Postman’s testing and assertion capabilities are essential tools in your API testing toolkit, making the process of validating APIs efficient and reliable.