App & Browser Testing Made Easy

Give your users a seamless experience by testing on 3000+ real devices and browsers. Don't compromise with emulators and simulators

Get Started free
Home Guide What is API Testing? (with Examples)

What is API Testing? (with Examples)

By Hamid Akhtar, Community Contributor -

API testing is a vital process that enables evaluating and validating the functionality, reliability, performance, and security of Application Programming Interfaces (APIs). 

APIs are like the connectors that allow different software systems to communicate and exchange data with each other. Think of it as ensuring that all the gears turn smoothly in a well-oiled machine.

What is API Testing?

Let’s take a real-life example to understand the importance of API testing. Imagine you’re using a mobile app for online shopping and want to check product availability at a specific store.

The app uses an API to connect with the store’s inventory system and fetch the data. If the API is tested properly, it might give incorrect information about the product’s availability, leading to customer satisfaction and loss of sales.

That’s why API testing is crucial. It ensures that APIs function correctly, adhere to specifications, and respond accurately.

Benefits of API Testing

  • API testing ensures that APIs work correctly, follow the rules, and give accurate responses. This builds trust in the API’s behavior.
  • It helps catch and fix bugs early, before they cause problems in the final product.
  • API testing enhances security by checking the API’s authentication, authorization, and data protection.
  • It validates performance metrics like response times, ensuring the system runs efficiently and can handle a lot of users.
  • API testing helps different software systems talk to each other smoothly, speeding up development and promoting teamwork.
  • By doing thorough API testing, organizations can deliver high-quality software, make customers happy, and reduce the risk of big failures.

By conducting thorough API testing, you can ensure seamless integration and efficient operation of software applications that rely on these APIs. It’s like ensuring all communication channels are clear, enabling different systems to work together flawlessly.

Types of API Testing

Here are API testing examples, including different types of tests:

  1. Unit Testing: Testing an API’s “login” function to authenticate user credentials.
  2. Functional Testing: Testing an e-commerce API to ensure proper shopping cart functionality.
  3. Performance Testing: testing an API to measure its speed and responsiveness under different loads.
  4. Security Testing: Testing an API to validate authentication and encryption methods for data protection
  5. Integration Testing: testing an API that integrates with a payment gateway for accurate payment processing).
  6. Load Testing: testing an API’s performance under high-user traffic
  7. Stress Testing: testing an API with large data or sudden spikes in user requests.
  8. Fuzz Testing: Trying different combinations of keys on a keyboard to find unexpected errors (e.g., sending random input to an API’s search function to check error handling).

By employing these various types of API testing, developers and organizations ensure their APIs’ correctness, performance, and security, delivering a reliable experience to users.

API Test Design 

1. Test Design Techniques

  • Equivalence Partitioning: Imagine grouping similar test scenarios together, like sorting clothes into different piles based on their colors.
  • Boundary Value Analysis: Picture testing the boundaries of valid inputs, such as checking a temperature sensor that works correctly at freezing and boiling points.
  • State Transition Testing: Think of testing how an API behaves as it moves between different states, just like a traffic light changing colors.

2. Test Coverage and Prioritization

  • Make sure to cover all the important parts of the API, like testing different features or functionalities. It’s like making sure you explore all the exciting areas of a new city you’re visiting.
  • Prioritize the most critical test scenarios first, focusing on what can have the biggest impact on the application and its users. It’s like dealing with the most urgent things on your to-do list before moving on to less important tasks.

3. Creating Effective Test Cases:

  • Create test cases that are easy to understand and follow, like writing clear instructions for a game you want to play with your friends.
  • Consider positive and negative scenarios, like thinking about different possibilities and outcomes in a fun puzzle or riddle.
  • Set up the conditions and environment for your tests, like preparing the stage and props for a play to ensure everything is ready for the actors.

4. Writing Test Code

  • Choose a programming language like Python and use libraries such as “requests” to interact with APIs. It’s like having a secret code language to communicate with the API.
  • You can write code that sends requests to the API and checks the responses. It’s like playing a game where you make moves and check if you got the desired results.

API Testing Code Examples

The following code snippet sends a POST request to the API endpoint, including the username and password data. After that, it verifies that the response status code is 200 and checks if the ‘success’ field in the response JSON is True.

import requests

url = ''
data = {'username': 'testuser', 'password': 'testpass'}

response =, data=data)

assert response.status_code == 200
assert response.json()['success'] == True

Remember,  API testing is all about exploring and ensuring the quality of APIs in a way that makes sense to you.

How to Perform API Testing?

To illustrate the analogy, consider API testing as charting a route on Google Maps.

  1. First, you understand your start and end points and any stops you want to make (similar to understanding the API and planning tests). 
  2. Then, you ensure your device has a good internet connection and GPS signal (setting up the environment). 
  3. You then follow the route Google Maps provides (executing the tests). 
  4. Finally, you check if you’ve arrived at your destination correctly (analyzing the results).
  5. If you took a wrong turn or encountered a roadblock, you would correct your course and continue (addressing issues and retesting).

Preparing for API Testing

  • Choose a testing tool: Similar to picking the right tool for a task, select a suitable testing tool like Postman, Advanced REST Client, or Curl to interact with the API and send requests.
  • Writing test code: Like preparing study material, you must write code to test the API. This code will help you send requests and validate the API’s responses.
  • Interacting with the API: Imagine having a conversation with the API. Use the testing tool to send different requests, such as HTTP/S, to check if the API functions correctly.
  • Validating the API’s output is like checking your answers after solving a problem. Include validations or assertions in your test cases to ensure that the API produces the expected output or results.

How to run API tests?

The most notable aspect here is Nightwatch.js, which stands out for its user-friendly syntax and intuitive commands that contribute to a seamless testing experience. Another benefit of Nightwatch.js is that it allows you to automate your API tests. This makes testing more efficient and saves time.

You can run tests automatically without doing them manually every time.

Nightwatch.js has a big community and many plugins available. This means you can get help, resources, and extensions to improve your testing.

1. Install Nightwatch.js: Install it first to use Nightwatch.js for API testing. Open your project folder in the terminal or command prompt and run the following command:

npm install nightwatch --save-dev
  • This command will download and install Nightwatch.js for your project.
  • Configure Nightwatch.js: After installing Nightwatch.js, you must configure it for your API tests. Create a file named nightwatch.conf.js in your project folder. In this configuration file, you can specify settings such as the test environment, API endpoints, authentication details, and other necessary information. Edit the configuration file according to your requirements.
  • Write test cases: Now, you can start writing your API test cases using Nightwatch.js. Create JavaScript files (usually with a .test.js extension) to define your test scenarios. Each test case represents a specific situation you want to test. You’ll make API requests and validate the responses inside the test case. Use Nightwatch.js commands and functions to interact with the API and perform assertions.
  • Run the tests: Once you have written your test cases, you can execute them using Nightwatch.js.

Open the terminal or command prompt, navigate to your project folder, and run the following command:

npx nightwatch
  • Nightwatch.js will run all the test cases defined in your project and provide the test results.
  • Analyze test results: Nightwatch.js will generate a detailed report after running the tests. You can review this report to see which tests passed and which failed. If a test fails, Nightwatch.js will provide error messages or other relevant information to help you understand what went wrong.
  • Debug and troubleshoot: If any of your tests fail or exhibit unexpected behavior, you must debug and troubleshoot the issues. Review the test case code and investigate potential causes of failure. Check the request and response details, verify the correctness of your assertions, and ensure that your test environment is properly set up. Identifying and addressing the problems can resolve issues and make your tests more reliable.
  • Repeat and maintain: API testing is an iterative process. As your API evolves or new features are added, you should continue running and maintaining your API tests. Regularly execute your tests to ensure ongoing functionality and reliability. Update your test cases if there are changes to the API. Following this iterative approach, you can continuously test and validate your API’s behavior.

7 Types of Bugs that API Testing Detects

API testing helps you find different problems in the software you are developing. Here are 7 common issues that API testing can help detect:

  1. Duplicate or missing functionality: API testing ensures that the software doesn’t have the same feature repeated or any missing features.
  2. Improper messaging: checks if the messages sent between different software parts are correct and complete.
  3. Error handling problems: looks for issues with how errors are handled in the software. It ensures errors are dealt with properly and don’t cause any unexpected troubles.
  4. Multi-threaded issues:  identify problems that can happen when different parts of the software are trying to do things simultaneously. It checks if the software can handle such situations without conflicts or errors.
  5. Security vulnerabilities: find security problems in the software, like weaknesses that could allow unauthorized access or attacks.
  6. Performance issues: checks how well the software performs. It looks for things like slow response times or excessive use of computer resources.
  7. Reliability problems: helps ensure the software is stable and reliable. It looks for issues like crashes or memory leaks that can cause the software to stop working unexpectedly.

How to Introduce API Testing in your Organization?

Introducing API testing in your organization is important for ensuring your software works well. Here’s how to go about it:

  • Understand the benefits of API testing: highlight improved software quality, reliability, faster development cycles, and better integration.
  • Assess current testing processes: Identify areas where API testing can bring value and convert existing tests.
  • Research and select API testing tools based on compatibility, ease of use, reporting, and community support.
  • Build knowledge and skills through training and resources.
  • Start with pilot projects to demonstrate value and generate enthusiasm.
  • Develop a test strategy and framework for API testing.
  • Automate tests for efficiency and repeatability.
  • Monitor and analyze results, and implement feedback loops.
  • Evolve and iterate, staying updated with emerging trends and tools.

Nightwatch.js is a great tool here because it’s simple to use. It automates test suites swiftly with minimal setup, offering readable and easily maintainable code. Its support for parallel testing enables efficient execution, saving time during the testing process.

What is API Testing? (with Examples)

  • You can write and understand test cases easily, even without a developer. 
  • It’s based on JavaScript, a popular programming language, so it’s easy for your developers to use.
  • Nightwatch.js also supports end-to-end testing. 
  • This means you can test the whole flow of your application, including how different parts work together. It helps ensure that your APIs are working properly.
  • If people have concerns, address them by explaining that Nightwatch.js is easy to use and will bring long-term benefits to the organization.
  • Finally, ask for support and resources to implement Nightwatch.js for API testing. This might include training, tools, and collaboration among teams.

Challenges of API Testing 

  • Identifying relevant test cases: For instance, if you’re testing a weather API, an important functionality to test could be retrieving the current temperature for a specific location. Focusing on testing this functionality ensures accurate and reliable weather data.
  • Handling response time: Consider an API for a ride-sharing service. You need to verify that the API responds quickly when users request a ride, ensuring a seamless and efficient experience for riders.
  • Managing API dependencies: Imagine testing an e-commerce API that relies on a payment gateway for processing transactions. It is important to include the payment gateway in your test cases to ensure the proper flow of payment data and successful transactions.
  • Testing in real-world scenarios: Suppose you’re testing a social media API. You should simulate scenarios like posting a new status, commenting on a post, and uploading images to ensure the API functions correctly and provides an interactive social media experience.
  • Validating data accuracy: You’re testing a banking API that retrieves account transaction history. You must validate that the API returns accurate and complete transaction details to ensure users can access reliable financial information.
  • Handling authentication and security: Consider an API for a healthcare application that handles sensitive patient data. Testing authentication mechanisms and verifying that data transmission occurs securely is crucial to protect patient confidentiality and privacy.
  • Ensuring compatibility: For example, if you’re testing a music streaming API, you should test its compatibility with various devices and operating systems such as iOS and Android, ensuring users can access and enjoy the music on different platforms.
  • Managing test environments: Imagine testing an API for a hotel booking platform. You need to set up test environments that replicate different scenarios, such as booking a room with different availability options,and  ensuring accurate testing of the API’s booking functionality.
  • Handling versioning: Suppose you’re testing a messaging API used by a chat application. As the application evolves, you must manage different versions of the API to maintain compatibility across different versions of the chat app and ensure smooth communication between users.
  • Automation and maintenance: Consider an API for a weather app that provides hourly forecasts. Automating the testing process using Nightwatch.js ensures the API’s data is consistently updated and accurate, even as new forecast data becomes available.

Solutions for Overcoming API Testing Challenges:

  • Prioritize critical functionalities for testing to ensure comprehensive coverage.
  • Include response time assertions in your test cases to validate performance.
  • Identify and include API dependencies in your test cases for thorough testing.
  • Simulate real-world scenarios and different environments to ensure API reliability.
  • Validate the accuracy and integrity of data returned by the API in your tests.
  • Address authentication and security concerns by testing different authentication mechanisms and implementing secure coding practices.
  • Perform cross-device and cross-platform testing to ensure compatibility.
  • Set up and manage test environments that closely mimic production to ensure accurate testing.
  • Implement proper version control and management to handle API evolution and backward compatibility.
  • Automate API testing using tools like Nightwatch.js to streamline test execution and reduce manual effort.

Closing Notes

In API testing, companies of all sizes and industries recognize its significance in ensuring software quality. Giants like Google, Meta, Amazon, Microsoft, Twitter, Netflix, and Uber have established themselves as industry leaders by leveraging robust API testing practices.

These companies rely on meticulous testing to guarantee their APIs’ seamless functionality, reliability, and security. By prioritizing API testing, they can deliver high-quality products and services, meet customer expectations, and stay ahead in the competitive tech landscape.


1. Difference between API testing and unit testing?

API testing differs from unit testing as it evaluates the holistic performance, reliability, functionality, and security of APIs, while unit testing focuses on validating individual code units within an application.

2. What is the best way to test API?

To ensure comprehensive API testing, it is best to combine manual testing, where requests are manually sent and responses inspected, with automated testing using frameworks like Nightwatch.js.

3. Which methodology is used for web API testing?

The widely adopted methodology for testing web APIs is REST (Representational State Transfer), which emphasizes standard HTTP methods and stateless data exchange in formats like JSON or XML.

4. Why is API used?

APIs facilitate seamless communication and data exchange between software systems or components.

They provide a standardized interface that enables integration, promotes interoperability, and empowers third-party developers to extend functionality.

Automation Testing Types of Testing

Featured Articles

What Is API Automation Testing?

Top 10 API Testing Tools in 2023

App & Browser Testing Made Easy

Seamlessly test across 20,000+ real devices with BrowserStack