Centralized error handler in Express - Mini Project: Build & Apply

Centralized error handler

📖 Scenario: You are building a simple Express server that handles user requests. To keep your code clean and easy to maintain, you want to create a centralized error handler that catches all errors in one place and sends a friendly error message to the client.

🎯 Goal: Build an Express server with a centralized error handler middleware that catches errors from routes and sends a JSON response with the error message and status code.

📋 What You'll Learn

Create an Express app with a basic route that throws an error

Add a configuration variable for the default error status code

Implement a centralized error handler middleware function

Use the error handler middleware in the Express app

💡 Why This Matters

🌍 Real World

Centralized error handling is used in real web servers to keep code clean and provide consistent error messages to users.

💼 Career

Knowing how to implement error handling middleware is essential for backend developers working with Express or similar frameworks.

1

Create Express app with a route that throws an error

Create an Express app by requiring express and calling express(). Then create a route GET / that throws an error with the message 'Something went wrong!'.

Express

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

// Your code here

Need a hint?

Use require('express') to import Express and express() to create the app. Define a route with app.get('/', (req, res) => { ... }) and throw an error inside it.

2

Add a default error status code variable

Create a constant variable called DEFAULT_ERROR_STATUS and set it to 500 to represent the default HTTP status code for server errors.

Express

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

const DEFAULT_ERROR_STATUS = 500;

// Your code here

Need a hint?

Use const DEFAULT_ERROR_STATUS = 500; to set the default error status code.

3

Implement centralized error handler middleware

Create an error handler middleware function called errorHandler that takes four parameters: err, req, res, and next. Inside it, send a JSON response with status set to err.status || DEFAULT_ERROR_STATUS and message set to err.message.

Express

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

const DEFAULT_ERROR_STATUS = 500;

function errorHandler(err, req, res, next) {
    // Your code here
}

Need a hint?

The error handler middleware has four parameters. Use res.status(...).json(...) to send the error status and message as JSON.

4

Use the error handler middleware in the Express app

Add the errorHandler middleware to the Express app using app.use(errorHandler) after all routes.

Express

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

const DEFAULT_ERROR_STATUS = 500;

app.get('/', (req, res) => {
    throw new Error('Something went wrong!');
});

function errorHandler(err, req, res, next) {
    res.status(err.status || DEFAULT_ERROR_STATUS).json({
        status: err.status || DEFAULT_ERROR_STATUS,
        message: err.message
    });
}

// Your code here

Need a hint?

Use app.use(errorHandler) after all routes to catch errors.