Health check endpoints in Express - Mini Project: Build & Apply

Health check endpoints

📖 Scenario: You are building a simple web server using Express.js. You want to add endpoints that let users check if the server is running and healthy.

🎯 Goal: Create two health check endpoints: /health that returns a simple status message, and /ready that returns a readiness message.

📋 What You'll Learn

Create an Express app instance called app

Add a GET endpoint /health that responds with JSON { status: 'ok' }

Add a GET endpoint /ready that responds with JSON { ready: true }

Make the app listen on port 3000

💡 Why This Matters

🌍 Real World

Health check endpoints help monitoring systems know if your server is running and ready to handle requests.

💼 Career

Many backend jobs require setting up health checks for services to ensure reliability and uptime.

1

Set up Express app

Create a variable called express by requiring the 'express' module. Then create an Express app instance called app by calling express().

Express

const express = require('express');
const app = express();
// Your code here

Hint

Use require('express') to import Express and then call it to create app.

2

Add /health endpoint

Add a GET endpoint /health to the app that sends a JSON response with { status: 'ok' }.

Express

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

// Add GET /health endpoint here
// Your code here

Hint

Use app.get('/health', (req, res) => { ... }) and inside send JSON with res.json({ status: 'ok' }).

3

Add /ready endpoint

Add a GET endpoint /ready to the app that sends a JSON response with { ready: true }.

Express

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

app.get('/health', (req, res) => {
  res.json({ status: 'ok' });
});

// Add GET /ready endpoint here
// Your code here

Hint

Use app.get('/ready', (req, res) => { ... }) and inside send JSON with res.json({ ready: true }).

4

Start the server

Make the app listen on port 3000 using app.listen.

Express

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

app.get('/health', (req, res) => {
  res.json({ status: 'ok' });
});

app.get('/ready', (req, res) => {
  res.json({ ready: true });
});

// Start server on port 3000
// Your code here

Hint

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

Practice

1. What is the main purpose of a health check endpoint in an Express app?

easy

A. To serve static files like images

B. To confirm the app is running and responsive

C. To handle user login requests

D. To connect to the database

Solution

1. Step 1: Understand what a health check endpoint does

   A health check endpoint is a simple route that tells if the app is alive and working.

2. Step 2: Compare options to this purpose

   Only To confirm the app is running and responsive matches this purpose. Other options are unrelated to health checks.

3. Final Answer:

   To confirm the app is running and responsive -> Option B

4. Quick Check:

   Health check = Confirm app running [OK]

Hint: Health check endpoints just say if app is alive [OK]

Common Mistakes:

• Thinking health check handles user data
• Confusing health check with static file serving
• Assuming it manages database connections

2. Which of the following is the correct syntax to create a simple health check GET route in Express?

easy

A. app.listen('/health', () => console.log('OK'));

B. app.post('/health', (req, res) => res.send('OK'));

C. app.get('/health', (req, res) => res.send('OK'));

D. app.use('/health', (req, res) => res.send('OK'));

Solution

1. Step 1: Identify the HTTP method for health check

   Health checks use GET requests to check status, so app.get is correct.

2. Step 2: Check syntax correctness

   app.get('/health', (req, res) => res.send('OK')); uses app.get with path '/health' and sends 'OK' response correctly.

3. Final Answer:

   app.get('/health', (req, res) => res.send('OK')); -> Option C

4. Quick Check:

   GET route with res.send('OK') = app.get('/health', (req, res) => res.send('OK')); [OK]

Hint: Health checks use GET routes with res.send('OK') [OK]

Common Mistakes:

• Using POST instead of GET
• Using app.use instead of app.get
• Trying to listen on a route path

3. What will be the output when a client sends a GET request to the following Express route?

app.get('/health', (req, res) => {
  res.status(200).send('Service is up');
});

medium

A. HTTP 200 OK with body 'Service is up'

B. HTTP 404 Not Found

C. HTTP 500 Internal Server Error

D. No response, request times out

Solution

1. Step 1: Analyze the route handler

   The route listens for GET on '/health' and sends status 200 with text 'Service is up'.

2. Step 2: Determine the HTTP response

   The client receives status 200 OK and the string 'Service is up' in the response body.

3. Final Answer:

   HTTP 200 OK with body 'Service is up' -> Option A

4. Quick Check:

   Status 200 + send text = HTTP 200 OK with body 'Service is up' [OK]

Hint: res.status(200).send('text') sends 200 OK with that text [OK]

Common Mistakes:

• Assuming 404 if route exists
• Confusing status codes
• Expecting no response or error

4. Identify the error in this health check endpoint code and choose the fix:

app.get('/health', (req, res) => {
  res.sendStatus(200);
  res.send('OK');
});

medium

A. Remove res.send('OK') because res.sendStatus ends response

B. Change res.sendStatus(200) to res.status(200)

C. Add next() call after res.sendStatus(200)

D. Change app.get to app.post

Solution

1. Step 1: Understand res.sendStatus behavior

   res.sendStatus(200) sends status and ends the response immediately.

2. Step 2: Identify why res.send('OK') causes error

   Calling res.send after response ended causes an error. So remove the second send.

3. Final Answer:

   Remove res.send('OK') because res.sendStatus ends response -> Option A

4. Quick Check:

   One response per request = Remove res.send('OK') because res.sendStatus ends response [OK]

Hint: Only send one response per request [OK]

Common Mistakes:

• Sending multiple responses in one route
• Confusing res.sendStatus and res.status
• Adding next() unnecessarily

5. You want your Express app's health check endpoint to return JSON with status and uptime in seconds. Which code correctly implements this?

hard

A. app.get('/health', (req, res) => {
res.json('status: ok, uptime: ' + process.uptime());
});

B. app.get('/health', (req, res) => {
res.send('status: ok, uptime: ' + process.uptime());
});

C. app.get('/health', (req, res) => {
res.status(200).send({ status: 'ok', uptime: process.uptime() });
});

D. app.get('/health', (req, res) => {
res.json({ status: 'ok', uptime: process.uptime() });
});

Solution

1. Step 1: Understand JSON response method

   res.json sends a JSON object with correct headers automatically.

2. Step 2: Check each option's response format

   app.get('/health', (req, res) => { res.json({ status: 'ok', uptime: process.uptime() }); }); sends a JSON object with keys 'status' and 'uptime' correctly. app.get('/health', (req, res) => { res.send('status: ok, uptime: ' + process.uptime()); }); sends a string, not JSON. app.get('/health', (req, res) => { res.status(200).send({ status: 'ok', uptime: process.uptime() }); }); sends an object with res.send but may not set JSON headers properly. app.get('/health', (req, res) => { res.json('status: ok, uptime: ' + process.uptime()); }); sends a string inside res.json, which is incorrect.

3. Final Answer:

   app.get('/health', (req, res) => { res.json({ status: 'ok', uptime: process.uptime() }); }); -> Option D

4. Quick Check:

   Use res.json with object for JSON response [OK]

Hint: Use res.json({key: value}) to send JSON response [OK]

Common Mistakes:

• Sending JSON as string instead of object
• Using res.send with object without headers
• Confusing res.json and res.send