1. Introduction

This tutorial aims to guide you on how to validate input data and handle errors in an Express.js application. Learning how to perform these tasks is crucial to ensuring the integrity of the data in your application and providing a seamless user experience.

By the end of this tutorial, you will learn:

• How to use Mongoose's built-in validation features
• Handling errors during data processing

Prerequisites: Basic understanding of JavaScript and familiarity with Express.js and Mongoose.

2. Step-by-Step Guide

2.1 Input Validation

Input validation is a process where you check if the data received from the client meets certain criteria. Mongoose has built-in validators for all the SchemaTypes it supports.

Consider a Mongoose schema for a user:

const userSchema = new mongoose.Schema({
  name: {
    type: String,
    required: true,
    minlength: 5,
    maxlength: 50
  },
  email: {
    type: String,
    required: true,
    minlength: 5,
    maxlength: 255
  }
});

In the above schema, we have defined some validation rules. For instance, the name field is required and should have a minimum length of 5 and a maximum length of 50 characters.

2.2 Error Handling

Errors can occur while validating input data or during other data processing tasks. It's essential to handle these errors gracefully to prevent application crashes and inform the user about any issues.

In Express, you can handle errors using middleware. Here is an example:

app.use((err, req, res, next) => {
  res.status(500).send('Something failed.');
});

The error handling middleware function has four arguments instead of the usual three. Express recognizes it as an error-handling middleware function.

3. Code Examples

3.1 Example 1: Input Validation

Below is a simple example demonstrating how to validate input data using Mongoose.

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/playground')
  .then(() => console.log('Connected to MongoDB...'))
  .catch(err => console.error('Could not connect to MongoDB...', err));

const userSchema = new mongoose.Schema({
  name: { 
    type: String, 
    required: true,
    minlength: 5,
    maxlength: 50
  },
  email: { 
    type: String, 
    required: true,
    minlength: 5,
    maxlength: 255
  }
});

const User = mongoose.model('User', userSchema);

async function createUser() {
  const user = new User({
    name: 'Alex',
    email: 'alex@example.com'
  });

  try {
    const result = await user.save();
    console.log(result);
  }
  catch (ex) {
    for (field in ex.errors)
      console.log(ex.errors[field].message);
  }
}

createUser();

3.2 Example 2: Error Handling

Here's how you can handle errors in Express.js.

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

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

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something failed.');
});

app.listen(3000, () => console.log('Listening on port 3000...'));

4. Summary

In this tutorial, we covered how to validate input data using Mongoose's built-in validation features and how to handle errors in Express.js. The next step would be to learn more about advanced validation techniques and error handling strategies.

Additional resources:

• Express.js Error Handling
• Mongoose Validation

5. Practice Exercises

1. Create a new schema for a blog post with validation rules and test it.
2. Implement error handling for a REST API endpoint in your Express.js application.
3. Try to create a custom validation rule for a Mongoose schema.

Solutions and explanations will depend on the specifics of the exercises you've been provided. Remember, practice is the key to mastering any skill. Happy coding!