Why Promise Rejection Reason Should Be An Error Object: A Refactoring Guide

Hey guys! Let's dive into a common refactoring issue in JavaScript: expecting promise rejection reasons to be Error objects. It might sound a bit technical, but don't worry, we'll break it down in a way that's super easy to understand. This guide will explain why using Error objects for promise rejections is important and how to implement this best practice in your code. We'll cover the problems that arise from using literals, the benefits of Error objects, and provide practical examples to help you refactor your code effectively.

Problem: Why Using Literals for Promise Rejection is a No-Go

So, what's the big deal about promise rejections? Well, when a promise fails, it rejects with a reason. The issue arises when we use simple values—like strings, numbers, or booleans—as these rejection reasons. While JavaScript allows this, it's not the best approach. Let's understand why.

The Pitfalls of Primitive Values

Using literals (primitive values such as strings, numbers, booleans, etc.) for promise rejection is generally discouraged because they don't provide enough context about the error. While it is syntactically valid to provide literals as a rejected promise value, it is considered best practice to use instances of the Error class or its subclasses instead. Imagine you get a rejection reason that's just a string like "Something went wrong." That's not very helpful, right? Where did it go wrong? Why? We need more details!

The use of literals for promise rejection can lead to several problems:

• Lack of Context: A simple string or number doesn't give you much information about what went wrong. This makes debugging a headache. Without proper context, it becomes challenging to trace the origin and nature of the error, leading to increased debugging time and potential oversights in error handling.
• Difficult Debugging: When you're staring at a vague error message, it's like searching for a needle in a haystack. You'll spend more time trying to figure out the root cause. The lack of specific information in primitive values makes it harder to pinpoint the exact location and circumstances of the error, turning the debugging process into a guessing game.
• Maintenance Issues: If you're working on a large project, these vague rejections can become a nightmare to maintain. Imagine someone else (or even you, months later) trying to understand what that string meant! In large codebases, consistency and clarity in error reporting are crucial for maintainability. Using literals introduces ambiguity and inconsistency, making the codebase harder to understand and maintain over time.

Code Examples of Non-Compliant Cases

Let's look at some code that shows the wrong way to do this:

new Promise(function(resolve, reject) {
  reject(); // Noncompliant: use Error object to provide rejection reason
});

new Promise(function(resolve, reject) {
  reject('Something went wrong'); // Noncompliant: use Error object instead of literal
});

In these examples, we're rejecting the promise with either nothing at all or just a simple string. Not very informative, huh?

Solution: Embrace the Power of Error Objects

So, what's the solution? The answer is to use Error objects (or instances of Error subclasses) for promise rejections. This is a best practice that brings a lot of advantages to the table.

Why Error Objects Are Your Best Friend

Using an instance of the Error class allows you to provide more meaningful information about the error. The Error class and its subclasses provide properties such as message and stack that can be used to convey useful details about the error, such as a description of the problem, the context in which it occurred, or a stack trace for debugging.

• Detailed Information: Error objects come with a message property, which lets you provide a clear description of what went wrong. This is way more helpful than a generic string. The message property allows developers to include specific details about the error, such as the cause, the expected input, or the state of the application at the time of the error.
• Stack Traces: One of the coolest things about Error objects is the stack property. It gives you a stack trace, showing you the path the code took to get to the error. This is gold for debugging! Stack traces provide a detailed call history, allowing developers to trace the execution path that led to the error. This is invaluable for identifying the source of the problem and understanding the sequence of events that triggered it.
• Consistency: Using Error objects makes your code consistent and easier to understand. Everyone knows what to expect when an error occurs. Consistency in error handling makes codebases more predictable and easier to navigate. When errors are consistently represented as Error objects, developers can rely on a standard structure to access error information, simplifying error analysis and debugging.

How to Use Error Objects in Promise Rejections

To fix your code provide an instanse of the Error class to the promise reject function. Let's see how to do it:

new Promise(function(resolve, reject) {
  reject(new Error('Network timeout'));
});

new Promise(function(resolve, reject) {
  reject(new Error('Something went wrong'));
});

See the difference? Now, we're rejecting with new Error('Network timeout') and new Error('Something went wrong'). Much clearer, right?

Subclasses of Error

For even more clarity, you can use subclasses of Error, like TypeError, ReferenceError, or even create your own custom error classes. This helps categorize errors and makes your code even more expressive. Subclasses of Error allow for more specific categorization of errors, making it easier to handle different types of errors in different ways. Custom error classes can be created to represent application-specific error conditions, providing a tailored approach to error management.

Practical Examples and Problem Locations

Now, let's look at some real-world examples from different parts of a codebase. These examples will help you understand where and how to apply this refactoring technique.

1. demo/src/components/FileInputForm.tsx

Imagine a file input form where things can go wrong—like the file being too large or the format being incorrect. Using Error objects here can provide specific messages like `