Are you tired of debugging tangled asynchronous code in JavaScript? Do fragmented error messages from failed promises make your development process a nightmare? Many developers struggle with managing multiple errors gracefully, leading to bloated code, frustrating user experiences, and endless hours of troubleshooting. But what if there was a built-in JavaScript feature designed to simplify this chaos?

Enter AggregateError. This powerful JavaScript error type bundles multiple individual errors into a single, comprehensive object. It’s an indispensable tool for anyone working with complex asynchronous operations or needing robust form validation.

The Pain Point: Traditional Error Handling

Before AggregateError, handling multiple rejections from asynchronous operations or collecting various validation failures was a cumbersome process. Consider a scenario where you’re making several API calls in parallel or validating multiple form fields. Without a unified error mechanism, you’re left with:

• Repetitive Code: Numerous try/catch blocks or individual error checks clutter your codebase.
• Fragmented User Feedback: Users might only see the first error encountered, leaving them guessing about other issues.
• Debugging Headaches: Pinpointing all root causes of failure across multiple promises becomes a tedious task.

For instance, imagine trying to fetch data from several APIs using Promise.any(). If all APIs fail, the traditional approach would involve complex logic to individually identify and log each rejection:

// ... (omitted previous example code for brevity)
// This traditional approach manually tries to catch and log each API error,
// which is inefficient and repetitive.

Similarly, for form validation, you’d typically throw an error on the first failed check, preventing the user from seeing all issues at once:

// ... (omitted previous example code for brevity)
// This method stops at the first error, providing incomplete feedback.

Embrace Clarity with AggregateError

AggregateError provides an elegant solution by consolidating all errors into one digestible object. It typically surfaces when using methods like Promise.any() when all promises in the iterable are rejected. However, you can also manually create and throw it for custom validation logic.

1. Handling Promise Rejections with Promise.any()

When Promise.any() is used, it resolves as soon as one of the promises succeeds. If, however, all promises fail, it throws an AggregateError. This error object contains a .errors property, which is an array of all the individual rejection reasons.

const fetchFromApi1 = () => Promise.reject(new Error("API 1 failed: Service unavailable"));
const fetchFromApi2 = () => Promise.reject(new Error("API 2 failed: Invalid authentication"));
const fetchFromApi3 = () => Promise.reject(new Error("API 3 failed: Timeout"));

async function fetchWithAggregateErrorHandling(apis, retries = 3, delay = 1000) {
  try {
    const data = await Promise.any(apis);
    console.log("Data fetched successfully:", data);
    return data;
  } catch (e) {
    if (e instanceof AggregateError) {
      console.error("All data sources failed with the following errors:");
      e.errors.forEach(err => console.error(`- ${err.message}`));

      if (retries > 0) {
        console.log(`Retrying... (${retries} attempts left)`);
        await new Promise(resolve => setTimeout(resolve, delay));
        return fetchWithAggregateErrorHandling(apis, retries - 1, delay);
      } else {
        console.error("All data sources failed after multiple attempts. Please try again later.");
        throw e; // Re-throw for higher-level error handling
      }
    } else {
      throw e; // Handle unexpected errors that aren't AggregateError
    }
  }
}

// Initiate fetching with retry logic
fetchWithAggregateErrorHandling([fetchFromApi1(), fetchFromApi2(), fetchFromApi3()]);

In this example, if all API calls fail, the catch block will receive a single AggregateError. We can then iterate through e.errors to display all rejection reasons, providing a complete picture of what went wrong. The retry logic is also cleanly encapsulated, attempting to resolve the issue before finally throwing the comprehensive error.

2. Enhanced Form Validation

You’re not limited to Promise.any(). You can manually create and throw an AggregateError for custom validation logic, such as form field validation. This allows you to run all validation checks, collect every failure, and then present them to the user in one clear message.

function validateUserProfile(user) {
  let validationErrors = [];

  if (!user.name || user.name.trim() === '') {
    validationErrors.push(new Error("User name is a mandatory field."));
  }
  if (!user.email || !user.email.includes('@')) {
    validationErrors.push(new Error("A valid email address is required."));
  }
  if (user.age === undefined || user.age < 18) {
    validationErrors.push(new Error("Users must be at least 18 years old to register."));
  }

  if (validationErrors.length > 0) {
    throw new AggregateError(validationErrors, "Multiple validation issues detected.");
  }
  return true; // Validation successful
}

try {
  validateUserProfile({ name: "", email: "invalid-email", age: 16 });
} catch (e) {
  if (e instanceof AggregateError) {
    console.error(e.message); // Output: "Multiple validation issues detected."
    e.errors.forEach(err => console.error(`- ${err.message}`));
    // Output:
    // - User name is a mandatory field.
    // - A valid email address is required.
    // - Users must be at least 18 years old to register.
  } else {
    console.error("An unexpected error occurred:", e.message);
  }
}

With this approach, every validation rule runs, and all errors are collected. The user receives a comprehensive list of what needs fixing, vastly improving the user experience and making debugging much more straightforward.

Key Benefits of Using AggregateError

• Streamlined Error Management: Consolidate diverse errors into a single, manageable object.
• Cleaner Codebase: Reduce the need for multiple try/catch blocks and complex error-handling logic.
• Enhanced User Experience: Provide users with a complete and clear picture of all issues at once, rather than one by one.
• Simplified Debugging: Easily access an array of all underlying errors, making it quicker to diagnose and fix problems.

Final Thoughts

AggregateError transforms chaotic asynchronous error handling into a structured, predictable process. Whether you’re building a complex data fetching mechanism or a robust form with multiple validation rules, adopting AggregateError will make your code cleaner, your debugging faster, and your user feedback more effective. Integrate this game-changing feature into your JavaScript projects and elevate your error handling to the next level.