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Exercise: Creating Conditional Middleware in Express

Objective

Develop an Express server that incorporates custom middleware to demonstrate conditional flow within the middleware stack. This exercise aims to expose students to the concept of controlling the execution chain based on certain conditions, such as validating request data.

Requirements

  1. Project Setup:

    • Initialize a new Node.js project and install Express.
    • Create an index.js file to hold your Express app logic.
    • Create a server.js file to import your Express app and run your server (by listening)

  2. Middleware Implementation:

    • Write a custom middleware function named validateRequestData.
    • This middleware should check for a specific property in the request body (e.g., isValid boolean flag).
    • If the property meets your condition (e.g., isValid is true), the middleware allows the request to proceed to the next handler.
    • If the condition is not met, the middleware should respond with a status code 400 and a message indicating invalid request data. Hint

  3. Setup Routes:

    • Implement a POST route /validate-data that uses your validateRequestData middleware.
    • If the request passes the middleware validation, respond with a status code 200 and a message indicating the data is valid.

  4. Server Configuration:

    • Configure your server to listen on port 3000.

End Cases

  • The server runs successfully without errors on port 3000.
  • Making a POST request to /validate-data with { "isValid": true } in the body proceeds past the middleware and receives a 200 status response indicating valid data.
  • Making a POST request to /validate-data without the isValid property or with { "isValid": false } results in a 400 status response indicating invalid request data.

Running the Tests

  1. Install Requirements
    • We have to start with installing our test packages as development dependencies
    • The packages we want to use are jest and supertest
    • We can install packages as dev dependencies by including a --save-dev flag when running the install command (like npm i --save-dev <your packages>)
    • With this information, what would the command to install jest and supertest as dev dependencies look like?
  2. Update the package.json scripts
    • If you've done things right, you should have a package.json file in your project with express and cors as dependencies, and jest and supertest as dev dependencies.
    • In the scripts section of the package.json, we want to update our test script to run jest instead of the default echo whatever and all that
  3. Run the tests
    • From here, if all has gone well, you can run the tests by running npm test
  4. Validate Correctness
    • Make sure that the tests pass, examine each test case and ensure that everything is passing, and if not, that you understand the errors that are happening and have a sense of how to fix it

Experiment

  • Test your endpoint with various payloads to see how your middleware affects the flow of request handling.
  • Try adding more conditions or checks to your middleware for additional validation.

Submission Guidelines

  • Push your completed project to a GitHub repository.
  • Ensure your package.json includes Express as a dependency.
  • Submit the URL to your GitHub repository.