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Express JS and EJS Handling Synchronous and Asynchronous Errors


Handling synchronous and asynchronous errors in an Express.js application is crucial for maintaining robustness and providing a good user experience. Here’s a detailed explanation of how to handle both types of errors with Express.js and EJS.

1. Synchronous Errors

Synchronous errors occur in the request-response cycle, typically within route handlers or middleware. These errors are thrown directly and need to be caught and managed.

Example:

app.get('/sync-error', (req, res, next) => {
    try {
        // Code that might throw an error
        throw new Error('This is a synchronous error');
    } catch (err) {
        next(err); // Pass the error to the error-handling middleware
    }
});

In this example, the try...catch block catches synchronous errors and passes them to the next middleware using next(err).

2. Asynchronous Errors

Asynchronous errors occur in operations that happen outside the immediate request-response cycle, such as during database queries, file operations, or network requests. These errors are not caught by try...catch blocks in the same way as synchronous errors.

Handling Errors in Asynchronous Code:

  1. Using async/await and try...catch:

    When using async/await, you can catch errors with try...catch:

    app.get('/async-error', async (req, res, next) => {
    try {
        // Simulate an asynchronous operation that might throw an error
        await someAsyncFunction();
        res.send('No errors');
    } catch (err) {
        next(err); // Pass the error to the error-handling middleware
    }
});

  2. Handling Errors in Callbacks:

    If you’re using callback-based asynchronous operations, you typically pass errors as the first argument to the callback:

    app.get('/callback-error', (req, res, next) => {
    someAsyncFunction((err, result) => {
        if (err) {
            return next(err); // Pass the error to the error-handling middleware
        }
        res.send(result);
    });
});

  3. Error Handling in Promise Chains:

    For promises, you can use .catch() to handle errors:

    app.get('/promise-error', (req, res, next) => {
    someAsyncPromiseFunction()
        .then(result => res.send(result))
        .catch(err => next(err)); // Pass the error to the error-handling middleware
});

3. Error-Handling Middleware

Regardless of whether the error is synchronous or asynchronous, you need to have error-handling middleware to catch and manage errors. The error-handling middleware should be defined after all other middleware and route handlers:

// Error-handling middleware
app.use((err, req, res, next) => {
    console.error(err.stack);
    res.status(err.status || 500).render('error', { error: err });
});

4. Providing User-Friendly Error Pages

You can create user-friendly error pages using EJS templates. For example, you might have an error.ejs file:

<!DOCTYPE html>
<html>
<head>
    <title>Error</title>
</head>
<body>
    <h1>Something went wrong!</h1>
    <p><%= error.message %></p>
    <pre><%= error.stack %></pre>
</body>
</html>

5. Handling Specific Error Types

You can customize error-handling based on the type or status of the error:

app.use((err, req, res, next) => {
    if (err.status === 404) {
        // Handle 404 errors
        res.status(404).render('404', { url: req.originalUrl });
    } else {
        // Handle other errors
        console.error(err.stack);
        res.status(err.status || 500).render('error', { error: err });
    }
});

Summary

  • Synchronous Errors: Handle with try...catch blocks and pass to the next middleware.
  • Asynchronous Errors: Handle using try...catch with async/await, error callbacks, or .catch() for promises.
  • Error-Handling Middleware: Define a middleware function with four parameters to catch and handle errors.
  • User-Friendly Pages: Use EJS templates to render error pages for a better user experience.
  • Specific Error Handling: Customize handling based on the type or status of the error.

By understanding and implementing these practices, you can effectively manage both synchronous and asynchronous errors in your Express.js application, ensuring a more robust and user-friendly experience.



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