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How To Test Rest Using Postman

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Getting Started with REST API Testing Using Postman

Postman is a powerful tool for testing REST APIs. It allows you to send requests, view responses, and manage your API requests in a user-friendly interface. This guide will show you how to use Postman to test REST APIs step-by-step, highlighting key concepts and practical examples.

1. Setting Up Your Environment

  • Installation: Download and install Postman from https://www.postman.com/.
  • Create a New Request: Open Postman and click on the “New” button to create a new request.

2. Building Your First Request

Example: GET Request to Fetch a User

  • Method Selection: Choose the HTTP method (e.g., GET, POST, PUT, DELETE) for your request. In this example, select “GET”.
  • Enter URL: In the address bar, enter the URL of the API endpoint you want to test. For example, https://api.example.com/users/1.
  • Headers: (Optional) Provide any necessary headers for authorization or content type. For instance: 
    Authorization: Bearer YOUR_API_TOKEN
    Content-Type: application/json

Sending Your Request

  • Click the “Send” button to execute your request.

3. Inspecting the Response

  • Response Body: View the response body in the “Body” tab of the Postman window. This will contain the data returned by the API.
  • Status Code: The status code (e.g., 200 OK) indicates the success or failure of the request.
  • Headers: Review the response headers to understand how the API has responded.

Example Response:

{
  "id": 1,
  "username": "john.doe",
  "email": "john.doe@example.com"
}

4. Creating and Using Collections

Collections in Postman are like containers for organizing your API requests. They’re crucial for managing and reusing test cases.

Creating a Collection:

  1. Click on “Collections” in the left sidebar.
  2. Click “Create Collection”.
  3. Give your collection a name (e.g., “User API”).

Adding Requests:

  1. Open the collection you want to work with.
  2. Click “Add Request” to add a new request to the collection.
  3. Give the request a descriptive name.

Benefits of Collections:

  • Organization: Keep your tests organized and manageable.
  • Reusability: Reuse your requests easily.
  • Run Tests Automatically: Run multiple requests in a sequence with the “Runner” feature.

5. Parameterizing Requests

Example: GET Request with Query Parameters

  • Replace a hardcoded value in the URL with a variable using the {variable_name} syntax. For example:
    • URL: https://api.example.com/users?page={page}&limit={limit}
  • Add Variables: Click on the “Variables” tab (near the Send button) and define your variables: 
    page: 1
    limit: 10
  • Using Variables: When you execute the request, Postman will substitute the variables with their defined values.

6. Asserting Expectations

Example: Verifying User Creation Status Code

  • Assertions: Postman allows you to add assertions to your requests to check for expected behaviors.
  • Adding an Assertion:
    1. Click the “Tests” tab.
    2. Add the following assertion: 
      pm.test("Status code is 201", function () {
        pm.response.to.have.status(201);
      });
  • Running Tests: When you send the request, Postman will execute the assertions and report the results.

7. Environmental Variables

Example: Using a Base URL for Multiple Requests

  • Create an environment named “My Environment”.
  • Add a variable named “baseUrl” with the value “https://api.example.com”.
  • Now, instead of hardcoding the base URL in every request, you can use {{baseUrl}}.

Benefits of Environments:

  • Flexible Testing: Easily switch between different environments (e.g., development, staging, production).
  • Sharing Settings: Share environment variables with other team members.

8. Mocking APIs

Postman allows you to mock API responses, which is useful for:

  • Testing before actual APIs are available.
  • Simulating specific scenarios.
  • Creating isolated tests.

Example: Mocking a User Retrieval API:

  1. Create a new Mock Server.
  2. Create a new mock request (e.g., GET /users/1).
  3. Define the mocked response.
  4. Use the mock URL in your Postman requests.

9. Advanced Techniques

  • Pre-Request Scripts: Add custom JavaScript code to modify request headers, parameters, or even make extra requests before sending your primary request.
  • Test Scripts: Write JavaScript code to perform additional tests after receiving the response.
  • Data-Driven Testing: Use data files (e.g., JSON, CSV) to test your API with multiple sets of data.

By mastering these techniques, you’ll be able to write more comprehensive and effective tests for your REST APIs in Postman.

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