Handling Requests and Responses

# CHAPTER 6

Handling Requests and Responses #

1. Introduction #

2. Learning Objectives #

• Extract query strings from the Request object (req.query).

• Understand and utilize HTTP Status Codes.

• Return structured data using res.json().

• Chain response methods elegantly.

3. Beginner-Friendly Explanation #

1. 1. The Request (req): You hand the waiter a highly specific order ticket: "I want a steak (the URL path), medium rare, with no salt (the Query Strings)."

1. 2. The Waiter (Express): Takes your ticket, reads the specific details, and gives it to the chef.

1. 3. The Response (res): The waiter brings back the food perfectly plated (JSON format) and hands you a receipt that says "Success: Order Complete" (Status Code 200). If they are out of steak, the waiter returns immediately and says "Sorry, Sold Out" (Status Code 404).

4. Query Strings (req.query) #

// URL: /api/search?keyword=express&sort=asc
app.get('/api/search', (req, res) => {
    
    // Express automatically parses everything after the '?' into an object
    const searchTerm = req.query.keyword; // "express"
    const sortOrder = req.query.sort;     // "asc"
    
    res.send(`Searching for ${searchTerm}, sorting by ${sortOrder}`);
});

5. Returning JSON Data #

app.get('/api/profile', (req, res) => {
    const userProfile = {
        name: "Alice",
        role: "Admin",
        isActive: true
    };
    
    // Express converts the JavaScript object into a JSON string!
    res.json(userProfile);
});

6. HTTP Status Codes #

• 200 OK: Everything went perfectly.

• 201 Created: You successfully created new data in the database.

• 400 Bad Request: The client sent invalid data (like an invalid email).

• 401 Unauthorized: The client is not logged in.

• 404 Not Found: The requested resource does not exist.

• 500 Server Error: The database crashed or the code broke.

7. Chaining Status Codes and JSON #

app.get('/api/products/:id', (req, res) => {
    const productId = req.params.id;
    
    if (productId === "99") {
        // ERROR: Send a 404 Status Code with an error message
        res.status(404).json({
            status: "error",
            message: "Product not found in the database"
        });
    } else {
        // SUCCESS: Send a 200 Status Code with the data
        res.status(200).json({
            status: "success",
            data: { id: productId, name: "Laptop" }
        });
    }
});

8. Backend Workflow: Standardizing Responses #

9. Best Practices #

• Use the return Keyword: Notice in the example above, if productId === "99", we send a 404 response. However, the function continues executing! To prevent Express from trying to send a second response (which crashes the app), always write return res.status(404).json(...) when dealing with errors.

10. Common Mistakes #

• Mixing up Params and Query:

• /api/users/5 -> Uses req.params.id (Requires a Route definition of /:id).

• /api/users?id=5 -> Uses req.query.id (Requires a Route definition of just /api/users).

11. Exercises #

1. 1. Explain the difference between res.send() and res.json(). Why is res.json() mandatory when building an API for a mobile application?

12. Coding Challenges #

• Challenge: Write a GET route for /api/weather. The route expects two query strings: city and units (e.g., ?city=London&units=metric). Extract these values. If the city is missing, return a 400 Bad Request JSON error. If both exist, return a 200 OK JSON response containing the simulated weather data.

13. MCQs with Answers #

14. Interview Questions #

• Q: Describe a standardized JSON response structure for a REST API. Why is it important for frontend developers that the backend maintains this consistent structure for both success and error responses?

• Q: Explain the purpose of HTTP Status Codes. Give an example of a situation where you would return a 401 Unauthorized versus a 403 Forbidden.

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #