Understanding Promise Rejection Handling in Node.js with Async/Await

As a JavaScript developer working on a Node.js application, it is essential to understand how to handle rejected promises when using the async/await syntax. Two effective ways to handle this include using a try/catch block and using process.on('unhandledRejection', handler).

Using a Try/Catch Block

Utilizing a try/catch block allows you to effectively handle rejected promises in async/await syntax. When an async function encounters a promise that doesn't resolve, it will throw an exception, which can be caught using a catch block.

Here is a simple example:

async function getData() {
  try {
    let data = await fetchData();
    console.log(data);
  } catch (error) {
    console.error('Error:', error);
  }
}

In this function, we first try to fetch some data using the fetchData() function. If the promise that fetchData() returns doesn't resolve—for example, if the fetch fails for some reason—then an exception is thrown, which we catch in the catch block.

Using process.on('unhandledRejection', handler)

Another way of dealing with unhandled promise rejections in Node.js is by using process.on('unhandledRejection', handler). This global event is triggered whenever a promise is rejected and no error handler is attached to the promise within a turn of the event loop.

Here's how to use it:

process.on('unhandledRejection', (reason, promise) => {
  console.log('Unhandled Rejection:', reason);
  // You can also do something to handle these rejections here
});

If any promise is rejected and isn't caught, our handler function gets called with the reason for the rejection and the promise that was rejected. This can be a useful way to catch bugs in your code or ensure that all errors are effectively handled.

Conclusion

While both methods for handling promise rejections in Node.js using async/await syntax can be useful, it's important to understand your application's needs and apply them appropriately. Both methods provide ways to handle and debug potential issues related to promise rejections effectively, leading to cleaner, more reliable code. Remember to always handle promise rejections, as unhandled promise rejections can lead to unexpected application crashes and other hard-to-debug problems.