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Step Up Your JS: A Comprehensive Guide to Intermediate JavaScript

Error Handling with Promises

Explore how to manage errors in asynchronous JavaScript using Promises. Understand the use of try/catch blocks, error callbacks in .then methods, and the cleaner .catch approach. This lesson helps you confidently handle failures like network issues or missing files in promise-based code, preparing you for more advanced async patterns.

We'll cover the following...

In this lesson, we’ll learn how to handle errors with Promises. Here’s the code we were using in the last lesson.

Javascript (babel-node)
onRequest((request, response) => 
    readFile(request.fileToRead, data => 
        writeFile(request.fileToWrite, data, status =>
            response.send(status)
        )    
    )
);

Let’s add error handling to this function. We’ll use a simple try/catch block to catch any possible errors.

Javascript (babel-node)
onRequest((request, response) => {
    try {
        readFile(request.fileToRead, data => 
            writeFile(request.fileToWrite, data, status =>
                response.send(status)
            )    
        );
    } catch(err) {
        console.log(err);
        response.send(err);
    }
});

Here’s the Promise-enabled code we wrote last lesson.

Javascript (babel-node)
onRequest((request, response) => 
    readFile(request.fileToRead)
    .then(data => writeFile(request.fileToWrite, data))
    .then(status => response.send(status))
);

Error Handling

Asynchronous operations generally require error handling in case something goes wrong (network issues, data not found, bad request, etc.). Promises have this built-in. They can take in a second callback that handles errors. It’ll only be called in the case of an error. ...