Request and response schemas in Express - Mini Project: Build & Apply

Request and Response Schemas in Express

📖 Scenario: You are building a simple Express server that handles user data. To keep your server organized and safe, you want to define clear rules for what the server expects to receive in requests and what it sends back in responses.

🎯 Goal: Create an Express server with a route /user that accepts a POST request with a JSON body matching a user schema, and responds with a JSON confirming the received data using a response schema.

📋 What You'll Learn

Create an Express app

Define a user data schema for the request body with name (string) and age (number)

Create a POST route /user that validates the request body against the schema

Send a JSON response confirming the received user data

💡 Why This Matters

🌍 Real World

Defining request and response schemas helps keep APIs clear and prevents errors by ensuring data is what the server expects.

💼 Career

Backend developers often define and validate request and response schemas to build reliable and maintainable APIs.

1

Setup Express app and user data schema

Create an Express app by importing express and calling express(). Then create a constant called userSchema that is an object with keys name set to "string" and age set to "number".

Express

const express = require('express');
const app = express();

// Your code here

Hint

Use require('express') to import Express and call express() to create the app. Define userSchema as an object with the exact keys and string values.

2

Add JSON body parsing middleware

Add middleware to the Express app to parse JSON request bodies by calling app.use(express.json()).

Express

const express = require('express');
const app = express();

const userSchema = {
  name: "string",
  age: "number"
};

// Your code here

Hint

Use app.use(express.json()) to enable JSON parsing for incoming requests.

3

Create POST /user route with request validation

Create a POST route /user using app.post. Inside the route handler, get req.body and check if name is a string and age is a number. If valid, respond with JSON containing message: "User received" and the user data. If invalid, respond with status 400 and JSON { error: "Invalid user data" }.

Express

const express = require('express');
const app = express();

const userSchema = {
  name: "string",
  age: "number"
};

app.use(express.json());

// Your code here

Hint

Use app.post('/user', (req, res) => { ... }). Inside, check types with typeof. Send success JSON with res.json or error with res.status(400).json.

4

Start the server on port 3000

Add code to start the Express server listening on port 3000 by calling app.listen(3000).

Express

const express = require('express');
const app = express();

const userSchema = {
  name: "string",
  age: "number"
};

app.use(express.json());

app.post('/user', (req, res) => {
  const user = req.body;
  if (typeof user.name === 'string' && typeof user.age === 'number') {
    res.json({ message: "User received", user });
  } else {
    res.status(400).json({ error: "Invalid user data" });
  }
});

// Your code here

Hint

Use app.listen(3000) to start the server on port 3000.

Practice

1. What is the main purpose of using request and response schemas in an Express app?

easy

A. To define and validate the shape of data sent and received

B. To speed up the server response time

C. To style the API responses with colors

D. To automatically generate HTML pages

Solution

1. Step 1: Understand the role of schemas

   Request and response schemas specify what data structure is expected, helping to validate inputs and outputs.

2. Step 2: Identify the main benefit

   This validation helps catch errors early and keeps the API clear and consistent.

3. Final Answer:

   To define and validate the shape of data sent and received -> Option A

4. Quick Check:

   Schema purpose = validation [OK]

Hint: Schemas check data shape to avoid errors [OK]

Common Mistakes:

• Thinking schemas improve speed directly
• Confusing schemas with styling tools
• Assuming schemas generate HTML automatically

2. Which of the following is the correct way to use a schema validation middleware in Express for a POST request body?

easy

A. app.get('/user', validateSchema, (req, res) => { ... })

B. app.post('/user', validateSchema, (req, res) => { ... })

C. app.post('/user', (req, res) => validateSchema(req.body))

D. app.use(validateSchema('/user'))

Solution

1. Step 1: Identify correct HTTP method and middleware usage

   Schema validation middleware should be used in the route handler chain, typically for POST requests when validating request bodies.

2. Step 2: Check syntax correctness

   app.post('/user', validateSchema, (req, res) => { ... }) correctly places the middleware function before the route handler for POST '/user'.

3. Final Answer:

   app.post('/user', validateSchema, (req, res) => { ... }) -> Option B

4. Quick Check:

   Middleware before handler for POST = B [OK]

Hint: Middleware goes between route and handler [OK]

Common Mistakes:

• Using GET instead of POST for body validation
• Calling middleware inside handler instead of chaining
• Using app.use incorrectly with route path

3. Given this Express route using Joi schema validation middleware:

const schema = Joi.object({ name: Joi.string().required() });
app.post('/data', (req, res, next) => {
  const { error } = schema.validate(req.body);
  if (error) return res.status(400).send(error.message);
  res.send('Valid data');
});

What will the response be if the client sends {} as JSON body?

medium

A. Valid data

B. 500 Internal Server Error

C. 400 Bad Request with error message

D. Empty response with status 200

Solution

1. Step 1: Analyze schema requirements

   The schema requires a 'name' string field that is mandatory.

2. Step 2: Check validation result for empty body

   Sending an empty object {} misses the required 'name', so validation fails and error is returned with status 400.

3. Final Answer:

   400 Bad Request with error message -> Option C

4. Quick Check:

   Missing required field = 400 error [OK]

Hint: Missing required fields cause 400 error [OK]

Common Mistakes:

• Assuming empty body passes validation
• Expecting 500 error on validation failure
• Thinking success message is sent regardless

4. Consider this Express route snippet with schema validation:

app.post('/item', (req, res) => {
  const { id, price } = req.body;
  if (typeof id !== 'number' || typeof price !== 'number') {
    res.status(400).send('Invalid data');
    return;
  }
  res.send('Item accepted');
});

What is the main bug in this code?

medium

A. It does not stop execution after sending 400 response

B. It uses wrong HTTP method for data submission

C. It does not parse JSON body before accessing req.body

D. It sends response twice causing server crash

Solution

1. Step 1: Check response flow after validation failure

   After sending 400 response, the code continues and sends 'Item accepted' response, causing multiple responses.

2. Step 2: Identify missing control flow

   There is no return or else to stop execution after sending error response.

3. Final Answer:

   It does not stop execution after sending 400 response -> Option A

4. Quick Check:

   Send response then stop = return [OK]

Hint: Return after res.send to stop code [OK]

Common Mistakes:

• Ignoring missing return after error response
• Assuming Express auto-stops after res.send
• Confusing HTTP method correctness

5. You want to create an Express route that accepts a JSON body with an array of user objects, each having a required 'email' string and optional 'age' number. Which Joi schema correctly validates this request body?

hard

A. Joi.object({ users: Joi.array().required() })

B. Joi.object({ email: Joi.string().required(), age: Joi.number() }).required()

C. Joi.array().items(Joi.string().email().required())

D. Joi.array().items(Joi.object({ email: Joi.string().email().required(), age: Joi.number() }))

Solution

1. Step 1: Understand the data shape

   The body is an array of user objects, each with required 'email' string and optional 'age' number.

2. Step 2: Match schema to data shape

   Joi.array().items(Joi.object({ email: Joi.string().email().required(), age: Joi.number() })) defines an array of objects with the correct fields and validations.

3. Final Answer:

   Joi.array().items(Joi.object({ email: Joi.string().email().required(), age: Joi.number() })) -> Option D

4. Quick Check:

   Array of objects with required email = A [OK]

Hint: Use array().items(object({...})) for arrays of objects [OK]

Common Mistakes:

• Using object schema instead of array for array data
• Validating array of strings instead of objects
• Missing required email validation