Dodging Pitfalls in Asynchronous JavaScript: Handling Promise Rejections

JavaScript’s rise in popularity in the web development world is undisputed. One tool in the JavaScript arsenal is the Promise, which, if not handled correctly, can trip up both fledgling and seasoned developers. Unhandled promise rejections are one such pitfall that often confounds less experienced developers. Here, we demystify this concept and lay out clear guidelines on handling promise rejections properly.

Promises and Errors in JavaScript

Promises are JavaScript objects that represent a future result of an asynchronous operation. An operation might succeed (resolve) or fail (reject), and the Promise object encapsulates that potential state.

Errors within Promises can arise due to a myriad of reasons—network failure, database non-response, incorrect data, and more. When these errors occur within a Promise, it leads to a Promise rejection.

The Misstep: Unhandled Promise Rejections

An all too common mistake among new JavaScript developers is failing to handle these promise rejections. Let’s consider an example:

function getDataFromDatabase() {
  return new Promise((resolve, reject) => {
    // simulating a database operation that fails
    reject(new Error('Database error'));
  });
}

getDataFromDatabase().then((data) => {
  console.log(data);
});

In this code, we’re simulating a database operation that fails, causing the promise to reject. However, in the .then() clause, we’re only handling the case where the Promise is resolved, not rejected. This unhandled Promise rejection can lead to problems.

Node.js Response to Unhandled Promise Rejections

Prior to Node.js v15.0.0, unhandled Promise rejections only resulted in a console warning and let the application keep running, potentially leading to hidden bugs and unpredictable behavior.

Starting from version 15.0.0, Node.js treats unhandled promise rejections as serious programming errors. The Node.js process will throw an ‘unhandledRejection’ event, and if not caught, will terminate the process.

Catching Errors: Promises and async/await

To prevent unhandled Promise rejections, developers must always handle promise rejections, either by including a .catch() clause in Promise chains or using a try/catch block with async/await.

For our previous example, correct handling of the rejection would look like this:

async function fetchDataAndLog() {
  try {
    const data = await getDataFromDatabase();
    console.log(data);
  } catch (err) {
    console.error('An error occurred:', err);
  }
}

fetchDataAndLog();

In the updated code, we added a .catch() clause. This clause is invoked if the Promise is rejected, handling the error gracefully, and avoiding the unhandled promise rejection.

For async/await, a different syntax is used. The async/await syntax makes asynchronous code appear more like synchronous code, allowing us to use the familiar try/catch syntax for error handling. If an error is thrown inside an async function, it results in the returned Promise being rejected, and you can catch that rejection with catch.

Here’s how you might use try/catch with async/await:

async function fetchDataAndLog() {
  try {
    const data = await getDataFromDatabase();
    console.log(data);
  } catch (err) {
    console.error('An error occurred:', err);
  }
}

fetchDataAndLog();

In this code, we await the getDataFromDatabase() function. If the Promise rejects, the error is thrown and caught in the catch block, preventing an unhandled Promise rejection.

Conclusion

Promises and async/await are powerful tools in a JavaScript developer’s toolbox, enabling elegant handling of asynchronous operations. It’s important to remember, however, that each Promise should either be resolved or rejected. If a Promise is rejected, that rejection should be handled. Failing to do so is a common mistake among junior developers. But with a firm understanding of Promises, async/await, and error handling in JavaScript, developers can avoid this pitfall, leading to more robust, reliable code.