In this session, we're delving into error handling within Node.js and Express.js. As a robot needs instructions for instances when an object cannot be found, our code similarly requires mechanisms for handling errors.

Error handling is a procedure that detects and manages errors during program execution. Unhandled errors, generated by Syntax Errors (breaches of JavaScript rules), and Runtime Errors (exceptions during execution), can disrupt programs — akin to a robot attempting to find a non-existent object.

The try statement examines a block of code for errors. Any existing errors are "caught" by the catch block, thus providing error management.

JavaScript
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1try {
2  // Attempts to print 'num'
3  console.log(num);
4} catch (err) {
5  // If an error occurs (e.g., `num` wasn't defined), it's caught, and an error message is printed
6  console.log(err.message);
7}

In this piece of code, instead of letting the program crash (due to the issue that num is not defined), we "catch" the error and handle it gracefully.

Effective error handling enhances user experience by utilizing appropriate HTTP status codes (e.g., HTTP 200 signifies success, HTTP 404 denotes a resource not found, HTTP 500 denotes an internal server error) within the API.

JavaScript
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1// Endpoint for retrieving user data by ID
2app.get('/api/user/:id', (req, res) => {
3  try {
4    const user = getUserById(req.params.id);
5    if (!user) {
6      // If the user was not found, return status 404
7      res.status(404).json({message: "User not found"});
8      return;
9    }
10    res.json(user);
11  } catch (err) {
12    // If an error occurs, a response with status code 500 is sent
13    res.status(500).json({ error: "Server error" });
14  }
15});

In this context, we use try/catch for handling server-side exceptions, and catch dispatches an HTTP status code 500 (Internal Server Error) for server errors.

In a full-stack application, both client and server errors require attention.

For the server, consider the following:

JavaScript
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1// API endpoint that attempts to update a user's details
2app.put('/api/user/:id', (req, res) => {
3  try {
4    const user = updateUser(req.params.id, req.body);
5    // rest of the code
6  } catch (err) {
7    // If an error occurs, an error response is sent
8    res.status(500).json({ error: 'Failed to update user' });
9  }
10});

On the client side (React with Axios), the catch block manages errors during the GET request.

JavaScript
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1import React, { useEffect } from 'react';
2import axios from 'axios';
3
4function App() {
5  useEffect(() => {
6    // This function will be triggered after the component is mounted
7    axios.get('/api/user/123')
8      .then(res => {
9        // Handle success
10      }).catch(err => {
11        // Error during GET request is handled here
12        console.error(err.message);
13      });
14  }, []);
15 
16  // Rest of the functional component code
17
18  return (
19    // Your JSX here
20  );
21}
22
23export default App;

Today, we have covered error handling, try/catch blocks, error handling in APIs, and a full-stack application. By avoiding application crashes and providing valuable feedback, proper error handling proves beneficial. Prepare for forthcoming practice tasks, where your learning is enhanced by practical application. Remember, to err is human; to handle is divine!