API Mocking with Mockoon

API Mocking with Mockoon


API mocking is an essential part of modern web development, allowing developers to simulate server responses and streamline the testing process. Mockoon is one of the simplest and most efficient tools for mocking APIs, offering an intuitive interface and a wide range of functionalities.

This blog post is a beginner-friendly guide on installing Mockoon on your laptop and configuring it to return common response codes like 200 (OK) and 400 (Bad Request), complete with a mock JSON body for a typical Customer API.

1: Installing Mockoon

  1. Go to the Mockoon official website (mockoon.com).
  2. Choose and download the installer for your operating system (Windows, MacOS, or Linux).
  3. Run the downloaded file and follow the installation instructions.
  4. Once the installation is complete, launch Mockoon to begin setting up your mock environment.

2: Configuring Mockoon for API Mocking

  1. Open Mockoon. Click on ‘Create a new environment’ to start setting up your mock API.
  2. Assign a port number for the mock server (e.g., 3000) ensuring it doesn’t conflict with other local servers.
  3. Save these environment settings to initiate the mock server.

3: Setting Up a Mockoon Customer API Endpoint

When configuring your environment in Mockoon, the creation and setup of routes play a crucial role in simulating the behavior of your actual APIs.

Here’s a detailed guide on how to effectively add and configure routes in your Mockoon environment:

Adding a New Route:

  • Start by launching Mockoon and accessing your desired environment.
  • To add a new route, simply click on the ‘Add Route’ button. This action creates a new route entry in your environment, which you can begin configuring according to your API specifications.

Setting Up a Typical Customer API Endpoint:

  • For a standard Customer API, you might name your endpoint as /api/customers. This endpoint could represent a collection of customer resources.
  • Enter /api/customers in the route path field. This designates the specific URL path that your mock service will listen to.

Choosing the Correct HTTP Method:

  • Different operations on your API will require different HTTP methods. In Mockoon, you can choose from methods like GET, POST, PUT, DELETE, etc., depending on the action your route is supposed to mimic.
  • For instance, a GET method is typically used for read operations. So, if your /api/customers endpoint is meant to retrieve a list of customers, set its method to GET.
  • For update operations, you might use POST or PUT. For example, to update a customer’s details, you could set up another route like /api/customers/{id} with the method POST or PUT.

Creating Multiple Endpoints for Different Operations:

  • In a realistic API, different endpoints handle different actions. Replicate this in your Mockoon environment by setting up multiple endpoints.
  • Each endpoint should follow the naming conventions and patterns of the system you are trying to mock. For instance, you might have /api/customers for listing customers, /api/customers/{id} for specific customer details, /api/customers with POST for creating a new customer, etc.
  • Ensure that each endpoint in Mockoon corresponds to an endpoint in your real API. This consistency is key for effective mocking and testing.

Configuring Responses:

  • For each route, configure the response that Mockoon should return. This could be a JSON object, plain text, or any other format that your API would normally send.
  • Customise status codes, headers, and body content to simulate different scenarios, like successful responses, errors, and edge cases.

Testing and Validation:

  • Once your routes are set up, test them using an API client like Postman or a browser, depending on the HTTP method.
  • Validate that the responses you receive are as expected and adjust your Mockoon configurations as necessary.

By thoroughly setting up your routes in Mockoon, complete with appropriate endpoints, HTTP methods, and response configurations, you create a robust mock environment. This environment is invaluable for development, testing, and understanding the behaviour of your APIs before they are deployed or while they are being developed.

Step 4: Configuring Response Codes and JSON Body

200 OK Response:

  • Create a new response under the route.
  • Set the status to 200, which indicates a successful request.
  • In the body, define a JSON object representing a typical customer. For example:
  "id": "12345",
  "firstName": "John",
  "lastName": "Doe",
  "dateOfBirth": "1980-01-01",
  "email": "johndoe@example.com",
  "mobilePhone": "+123456789"

400 Bad Request Response:

  • Add another response for scenarios where the request is malformed.
  • Set the status to 400.
  • In the body, provide a simple error message, such as
  "error": "Bad request. Please check your data."

Step 5: Testing Your Mock API

  1. Use a tool like Postman, or directly from your ASP.NET application, to send requests to your Mockoon server (e.g., http://localhost:3000/api/customers).
  2. Test different scenarios to ensure your mock server correctly returns the 200 and 400 response codes with the corresponding JSON bodies.


Mockoon provides a straightforward and powerful platform for ASP.NET developers to mock APIs, significantly simplifying the testing and development process.

By creating mock responses for typical API interactions, you can ensure your application is well-prepared to handle real-world scenarios, ultimately leading to more robust and reliable software.

With Mockoon, simulating a Customer API or any other service becomes a hassle-free task, enabling you to focus more on developing the core functionalities of your applications.


Hi, my name is Stephen Finchett. I have been a software engineer for over 30 years and worked on complex, business critical, multi-user systems for all of my career. For the last 15 years, I have been concentrating on web based solutions using the Microsoft Stack including ASP.Net, C#, TypeScript, SQL Server and running everything at scale within Kubernetes.